SC 13D/A

QuickLinks -- Click here to rapidly navigate through this document

SECURITIES AND EXCHANGE COMMISSION
WASHINGTON, DC 20549

SCHEDULE 13D/A
(Rule 13d-101)

INFORMATION TO BE INCLUDED IN STATEMENTS FILED PURSUANT TO RULE 13d-1(a) AND AMENDMENTS THERETO FILED PURSUANT TO RULE 13d-2(a)
(Amendment No. 4)

Western Gas Resources, Inc.
(Name of Issuer)

Common Stock
(Title of Class of Securities)

958259 10 3
(CUSIP Number)

Fiona E. Arnold
Associate General Counsel
Western Gas Resources, Inc.
1099 18^th Street, Suite 1200
Denver, CO 80202
(303) 450-8439
(Name, Address and Telephone Number of Person Authorized
to Receive Notices and Communications)

(Date of Event which Requires Filing of this Statement)

        If the filing person has previously filed a statement on Schedule 13G to report the acquisition which is the subject of this Schedule 13D, and is filing this schedule because of Rule 13d-1(e), 13d-1(f) or 13d-1(g), check the following box    o.

(Continued on following page(s))
Page 1 of 7 Pages

CUSIP No.    958259 10 3 13D Page 2 of 7 Pages

(1)	Names of Reporting Persons. Walter L. Stonehocker S.S. or I.R.S. Identification Nos. of Above Persons (entities only)
(2)	Check the Appropriate Box if a Member of a Group*			(a)  o
				(b)  o
(3)	SEC Use Only
(4)	Source of Funds* OO
(5)	Check if Disclosure of Legal Proceedings is Required Pursuant to Items 2(d) or 2(e)
				o
(6)	Citizenship or Place of Organization United States
NUMBER OF SHARES		(7)	Sole Voting Power 1,258,426 (1)
BENEFICIALLY
OWNED BY EACH REPORTING		(8)	Shared Voting Power 0
PERSON WITH
		(9)	Sole Dispositive Power 1,258,426 (1)
		(10)	Shared Dispositive Power 0
(11)	Aggregate Amount Beneficially Owned by Each Reporting Person 2,507,028 (1)
(12)	Check if the Aggregate Amount in Row (11) Excludes Certain Shares*
				o
(13)	Percent of Class Represented by Amount in Row (11) 7.6%
(14)	Type of Reporting Person* IN
(1) Includes 667 shares which reporting person has right to acquire through the exercise of stock options. *SEE INSTRUCTIONS BEFORE FILLING OUT!

        This 13D/A Amendment No. 4 hereby amends and restates in its entirety the Schedule 13D previously filed by Walter L. Stonehocker with respect to his beneficial ownership of common stock of Western Gas Resources, Inc.

ITEM 1. Security and Issuer.

        This Statement on Schedule 13D/A (this "Schedule 13D/A") relates to shares of common stock, par value $0.01 per share (the "Common Stock") of Western Gas Resources, Inc., a Delaware corporation (the "Issuer"). The Issuer's principal executive offices are located at 1099 18^th Street, Suite 1200, Denver, Colorado 80202.

ITEM 2. Identity and Background.

(a)

This Schedule 13D/A is being filed by Mr. Walter L. Stonehocker;



(b)

Mr. Stonehocker's business address is 15600 Holly, Brighton, Colorado 80602.



(c)

Mr. Stonehocker's principal occupation is retired Senior Vice President.



(d)

During the last five years, Mr. Stonehocker has not been convicted in a criminal proceeding (excluding traffic violations or similar misdemeanors).



(e)

During the last five years, Mr. Stonehocker has not been a party to a civil proceeding of a judicial or administrative body of competent jurisdiction that would have made him subject to any judgment, decree or final order enjoining future violations of, or prohibiting or mandating activities subject to, Federal or State securities laws or finding any violation with respect to such laws.



(f)

Mr. Stonehocker is a citizen of the United States.

ITEM 3. Source and Amount of Funds or Other Consideration.

        WGP, Inc., a Colorado corporation ("WGP") that was 18.35% owned by Mr. Stonehocker prior to its dissolution on May 1, 1991, acquired 4,014,907 shares of Common Stock of the Issuer pursuant to a subscription agreement between the Issuer and WGP that was accepted by the Issuer on November 3, 1989 and became effective on December 8, 1989 concurrently with the commencement of the public offering of shares of Common Stock by the Issuer (the "Public Offering"). WGP acquired the shares of Common Stock primarily for investment purposes.

        The Issuer was formed in October 1989 to acquire a majority interest in Western Gas Processors, Ltd., a Colorado limited partnership (the "Partnership") that merged out of existence on May 1, 1991. The Issuer became the sole general partner of the Partnership on December 8, 1989, and, from that date through May 1, 1991, conducted all of its business through the Partnership. Upon completion of the Public Offering on December 15, 1989, the Issuer owned approximately 51% of the Partnership, including 15% acquired with the net proceeds from the Public Offering. With additional net proceeds received by the Issuer from the exercise by certain underwriters of an over-allotment option to purchase additional shares of Common Stock granted to such underwriters in connection with the Public Offering, the Issuer subsequently increased its ownership of the Partnership to approximately 52%.

        On October 8, 1990, the Issuer announced that it would propose a restructuring (the "Restructuring") after which it would own all outstanding interests in the Partnership. Mr. Stonehocker subsequently entered into an Agreement and Plan of Restructuring dated as of April 2, 1991 (the "Restructuring Agreement") among the Issuer, the Partnership and the "Founders" (consisting of Mr. Stonehocker, WGP, Heetco, Inc. NV, a Nevada corporation, Dean Phillips, Inc., a Missouri corporation, Sauvage Gas Company, a Colorado corporation, Sauvage Gas Service, Inc., a Nebraska

Page 3 of 7 Pages

corporation, Brion G. Wise, Bill M. Sanderson, Dean Phillips, Ward Sauvage, and, subject to the limitations set forth therein, Lanny F. Outlaw). The Restructuring Agreement provided that (i) the Founders would transfer (the "Transfer") to the Issuer all units of limited partner interest ("LP Units") in the Partnership owned by them, other than LP Units designated as $1.80 Cumulative Participating Preference Units ("PPUs"), in exchange for an equal number of shares of Common Stock and (ii) the Partnership would merge into the Issuer pursuant to a plan of merger (the "Merger") providing that all LP Units not owned by the Issuer after the Transfer (consisting of PPUs) would be converted into an equal number of shares of Common Stock.

        The Restructuring was consummated on May 1, 1991 after the stockholders of the Issuer and the limited partners of the Partnership voted to approve the transaction, and the Partnership ceased to exist. On May 1, 1991 pursuant to the Transfer, the 4,471,969 LP Units owned by WGP were assigned, transferred and delivered to the Issuer in exchange for an equal number of shares of Common Stock. After the Restructuring was consummated, WGP was dissolved on May 1, 1991, and Mr. Stonehocker became the beneficial owner of a number of shares of Common Stock equal to his percentage ownership of WGP multiplied by the number of shares of Common Stock owned by WGP, and WGP ceased to be the beneficial owner of any Common Stock. Mr. Stonehocker acquired beneficial ownership of 1,557,759 shares of Common Stock pursuant to the dissolution of WGP.

        Also, on May 1, 1991, pursuant to the Merger, 26,500 PPUs held jointly by Mr. Stonehocker and his wife, Mrs. Roswitha Stonehocker, were converted into an equal number of shares of Common Stock. Mr. And Mrs. Stonehocker subsequently purchased, as joint tenants, an additional 3,500 share of Common Stock in an open market transaction.

        Mr. Stonehocker may also be deemed the beneficial owner of 1,384,379 shares of Common Stock owned by his wife and 127,820 shares of Common Stock owned by WGP Stock Trust (a trust for the benefit of the Stonehockers' children and of which Mrs. Stonehocker is the trustee), each of which acquired beneficial ownership of such shares upon the dissolution of WGP. Mr. Stonehocker may also be deemed to be the beneficial owner of 3,183 shares of Common Stock beneficially owned by one of his sons.

        On May 17, 2002, Mr. Stonehocker was issued, in his capacity as a member of the Board of Directors of the Issuer, options to purchase 2,000 shares of Common Stock, 667 of which are vested and the underlying shares deemed beneficially owned.

        On May 19, 2003, Mr. Stonehocker was issued, in his capacity as a member of the Board of Directors of the Issuer, options to purchase 2,000 shares of Common Stock, none of which are currently vested.

ITEM 4. Purpose of Transaction.

        Since the acquisitions referred to in Item 3 above, the following shares owned directly by or in which Mr. Stonehocker may be deemed to have a beneficial interest have been disposed of:

(a)

300,000 shares of Common Stock held directly by Mr. Stonehocker pursuant to an underwritten registration statement;



(b)

30,000 shares held jointly by Mr. Stonehocker and Mrs. Stonehocker in a series of open market transactions;



(c)

212,000 shares of Common Stock held by Mrs. Stonehocker in a series of open market transactions; and



(d)

54,780 shares of Common Stock held by the WGP Stock Trust were gifted to another party.

Page 4 of 7 Pages

        Mr. Stonehocker intends to hold the remainder of his shares of Common Stock primarily for investment purposes. Mr. Stonehocker has no specific plans or proposals which relate to or would result in any of the actions or transactions described in paragraphs (a)—(j) of Item 4 to Schedule 13D.

ITEM 5. Interests in Securities of the Issuer.

(a)

Mr. Stonehocker is the beneficial owner of an aggregate of 2,507,028 shares of Common Stock constituting 7.6% of the shares of Common Stock of the Issuer outstanding on March 28, 2003.



(b)

Mr. Stonehocker possesses the sole power to vote or to direct the vote, and the sole power to dispose or direct the disposition of 1,258,426 shares of Common Stock in the Issuer of which he is the beneficial owner.



(c)

None.



(d)

Not applicable.

ITEM 6. Contracts, Arrangements, Understandings or Relationships with Respect to Securities of the Issuer.

        The issuer has granted Mr. Stonehocker 4,000 options to purchase shares of Common Stock of the issuer, 2,000 of which were issued pursuant to a stock option agreement dated May 17, 2002 and the remaining 2,000 of which were issued pursuant to a stock option agreement dated May 19, 2003. The options vest in annual one-third increments commencing on the first anniversary of the respective grant date. The options expire on the earlier of five years from the date of vesting or ten years from the grant date.

ITEM 7. Material to be Filed as Exhibits.

(1)

Subscription Agreement, effective December 8, 1989, between WGP, Inc. and Western Gas Resources, Inc. (previously filed as exhibit 10.31 to Western Gas Resources, Inc.'s Registration Statement on Form S-4, Registration No. 33-39588, dated March 27, 1991 and incorporated herein by reference).



(2)

Agreement and Plan of Restructuring, dated as of April 2, 1991, by and among Western Gas Resources, Inc., Western Gas Processors, Ltd. and Mr. Stonehocker, WGP, Inc., Heetco, Inc., Dean Phillips, Inc., Sauvage Gas Company, Sauvage Gas Service, Inc., Brion G. Wise, Bill M. Sanderson, Dean Phillips, Ward Sauvage and Lanny F. Outlaw (previously filed as exhibit 10.10 to Western Gas Resources, Inc.'s Registration Statement on Form S-1, Registration No. 33-43077, dated November 14, 1991 and incorporated herein by reference).



(3)

Stock Option Agreement, dated May 17, 2002, by and between Western Gas Resources, Inc. and Mr. Walter L. Stonehocker.



(4)

Stock Option Agreement, dated May 19, 2003, by and between Western Gas Resources, Inc. and Mr. Walter L. Stonehocker.

Page 5 of 7 Pages

SIGNATURE

        After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

July 11, 2003 (Date)

/s/  WALTER L. STONEHOCKER (Signature)

Walter L. Stonehocker (Name/Title)

Page 6 of 7 Pages

Index Exhibit

Exhibit Number		Description
(1	)	Subscription Agreement, effective December 8, 1989, between WGP, Inc. and Western Gas Resources, Inc. (previously filed as exhibit 10.31 to Western Gas Resources, Inc.'s Registration Statement on Form S-4, Registration No. 33-39588, dated March 27, 1991 and incorporated herein by reference).
(2	)	Agreement and Plan of Restructuring, dated as of April 2, 1991, by and among Western Gas Resources, Inc., Western Gas Processors, Ltd. and Mr. Stonehocker, WGP, Inc., Heetco, Inc., Dean Phillips, Inc., Sauvage Gas Company, Sauvage Gas Service, Inc., Brion G. Wise, Bill M. Sanderson, Dean Phillips, Ward Sauvage and Lanny F. Outlaw (previously filed as exhibit 10.10 to Western Gas Resources, Inc.'s Registration Statement on Form S-1, Registration No. 33-43077, dated November 14, 1991 and incorporated herein by reference).
(3	)	Stock Option Agreement, dated May 17, 2002, by and between Western Gas Resources, Inc. and Mr. Walter L. Stonehocker.
(4	)	Stock Option Agreement, dated May 19, 2003, by and between Western Gas Resources, Inc. and Mr. Walter L. Stonehocker.

Page 7 of 7 Pages

QuickLinks

ITEM 1. Security and Issuer.
ITEM 2. Identity and Background.
ITEM 3. Source and Amount of Funds or Other Consideration.
ITEM 4. Purpose of Transaction.
ITEM 5. Interests in Securities of the Issuer.
ITEM 6. Contracts, Arrangements, Understandings or Relationships with Respect to Securities of the Issuer.
ITEM 7. Material to be Filed as Exhibits.

Index Exhibit

EX-3

QuickLinks -- Click here to rapidly navigate through this document

Exhibit 3

WESTERN GAS RESOURCES, INC.
2002 NON-EMPLOYEE DIRECTORS' STOCK OPTION AGREEMENT

        THIS AGREEMENT, made as of May 17, 2002, by and between Western Gas Resources, Inc. (hereinafter called the "Corporation"), a Delaware corporation, and Walter L. Stonehocker, a non-employee director of the Corporation (hereinafter called the "Optionee").

RECITALS:

        A.    The Optionee is eligible as a non-employee director of the Corporation to participate in the Western Gas Resources, Inc. 2002 Non-Employee Director's Stock Option Plan (the "Plan").

        B.    The Board of Directors of the Corporation considers it desirable and in the Corporation's best interests that the Optionee be given an opportunity to purchase shares of its Common Stock in furtherance of the Plan to provide incentive for the Optionee to remain as a director of the Company and to promote the success of the Corporation.

        NOW THEREFORE, in consideration of the premises, it is agreed as follows:

        1.    Grant of Option.    The Corporation hereby grants as of May 17, 2002 (the "Grant Date") to the Optionee the right, privilege and option to purchase 2,000 shares of the Common Stock par value $0.10 (the "Common Stock") of the Corporation, at a purchase price of Thirty Seven Dollars and Eighty Two 100ths ($37.82) per share in the manner and subject to the conditions hereafter provided. Said purchase price is not less than the Fair Market Value (as that term is defined in the Plan) of the shares of Common Stock of the Corporation at the time this option was granted.

        2.    Period of Exercise of Option.    This Option may be exercised in whole or in part, or in installments, from time to time, with respect to the shares covered hereby, in the amounts and at the times specified below. The Option or any portion thereof, once it becomes exercisable as specified below, shall remain exercisable until it shall expire in accordance with the provisions of this Agreement.

        (a)   Notwithstanding anything herein to the contrary, no Option or portion thereof granted under this Agreement may be exercised after the earlier of (i) five (5) years after the date the Optionee has the right to exercise such Option or portion thereof, in accordance with paragraph 2(b) below; or (ii) ten (10) years after the Option is granted.

        (b)   Except as expressly provided in Section 2(e), below, an Optionee shall become entitled to exercise that portion of the Option and to purchase the percentage of the Common stock subject to the Option in accordance with the following schedule:

        (1)   Commencing one (1) year from the Grant Date, the Optionee shall have the right to exercise and purchase thirty-three and one-third percent (33-1.3%) of the Option subject to the Option.

        (2)   Commencing two (2) years from the Grant Date, the Optionee shall have the right to exercise and purchase thirty-three and one-third percent (33-1.3%) of the Option subject to the Option.

        (3)   Commencing three (3) years from the Grant Date, the Optionee shall have the right to exercise and purchase thirty-three and one-third percent (33-1.3%) of the Option subject to the Option.

        The Optionee's right to purchase Shares subject to the Option shall be cumulative, so that three (3) years from the Grant Date, the Optionee shall be entitled to exercise one hundred percent (100%) of the Option and to purchase all of the Common Stock subject to the Option, subject to all of the provisions of this Agreements.

        (c)   Except as provided in Sections 2(d) and 2(e), an Optionee may exercise an Option only if, at the time such Option is exercised, such Optionee is a director of, and has continuously since the grant of the Option, been a director of the Corporation or any subsidiary, parent, or predecessor of the Corporation.

        (d)   If an Optionee ceases to be a director for any reason other than (i) his or her death or disability; or (ii) his or her discharge for dishonesty or commission of a crime, the Optionee may, within three (3) months thereafter, and subject to provisions of Sections 2(a), (b) and (c), exercise the Option to the extent that the Option was exercisable as of the date of the Optionee ceased to be a director. All unexercised Options, or portions thereof, shall terminate, be forfeited, and shall lapse upon expiration of said three (3) month period, or immediately if the Optionee ceases to be a director of the Corporation for any of the reasons set forth in (ii), above.

        (e)   If an Optionee dies or becomes disabled while he is a director of the Corporation or ceases to be a director as a result of disability, all of the Options granted to such employee shall become one hundred percent (100%) exercisable, without regard to the provisions of Section 2(b), above. In such event, the Options may be exercised by the disabled director, or the person or persons to whom his or her rights under the Option shall pass by will, or by the applicable laws of descent and distribution; provided, however, that no such Option may be exercised after 180 days from such directors' s date of death, or the date Optionee ceases to be a director as a result of disability, whichever is applicable. Upon expiration of said period, all unexercised Options, or portions thereof, shall terminate, be forfeited, and shall lapse.

        (a)   (f) Notwithstanding the provision of Section (b), above, in the event (1) there is a "Change of Control" of the Corporation; and (2) the Optionee is removed as a director of the Corporation without cause, then all of the options granted to such Optionee under this Agreement shall become one hundred percent (100%) exercisable, subject to the other provisions of this Section 2. For these purposes, a Change of Control of the Corporation shall mean (i) the acquisition by any person or persons acting in concert (including corporations, partnerships, associations or unincorporated organizations), of legal ownership or beneficial ownership (within the meaning of Rule 13d-3, promulgated by the Securities and Exchange Commission and now in effect under the Securities Exchange Act of 1934 (as amended), of a number of voting shares of capital stock of the Corporation greater than either 30% or the number of voting shares of capital stock of the Corporation that are then owned, beneficially (as defined above), by Brion G. Wise, Bill M. Sanderson, Walter L. Stonehocker, Dean Phillips, Ward Sauvage, their immediate families and the companies through which they and their immediate families hold ownership in the Corporation ("the Founders"), whichever is higher; (ii) a merger or consolidation of the Corporation or any of its subsidiaries other than a merger or consolidation immediately following which the directors of the Corporation prior thereto constitute a majority of the of the board of the surviving company or parent thereof; (iii) a change in the majority of the Board pursuant to an actual or threatened proxy contest; or (iv) a sale of substantially all of the Corporation's assets.

        3.    Method of Exercise.    To exercise an Option, the Optionee, or his or her successors, shall give written notice to the Treasurer of the Corporation, at the Corporation's principal office, accompanied by full payment of the Common Stock being purchased. If the Option is exercised by the successor of the Optionee, following his or her death, proof shall be submitted, satisfactory to the Board, of the right of the successor to exercise the Option. The Corporation shall not be required to transfer or deliver any certificate or certificates for shares purchased upon any such exercise of said option: (a) until after compliance with all then applicable requirements of law; and (b) prior to admission of such shares to listing on any stock exchange on which the stock may then be listed. In no event shall the Corporation be required to issue fractional shares to the Optionee.

2

        4.    Limitation Upon Exercise.    The option is not transferable by the Optionee otherwise than by will or the laws of descent and distribution and is exercisable, during the lifetime of Optionee, only by the Optionee.

        5.    Limitation Upon Transfer.    Except as otherwise provided hereto, the option and all rights granted hereunder shall not be transferred by the Optionee, and may not be assigned, pledged, or hypothecated in any way and shall not be subject to execution, attachment or similar process. Upon any attempt to transfer the option, or to assign, pledge, hypothecate or otherwise dispose of such Option or of any rights granted hereunder, contrary to the provisions hereof, or upon the levy of any attachment or similar process upon such option or such rights, such option and such rights shall immediately become null and void.

        6.    Stock Adjustment.    In the event of any change in Common Stock of the Corporation, by reason of a stock split, stock dividend, recapitalization, exchange of shares, or other transaction, the number of shares remaining subject to the option and the option price per share shall be appropriately adjusted by the Board of Directors.

        7.    Corporate Reorganization.    If there shall be any capital reorganization or consolidation or merger of the Corporation with another corporation or corporations, or any sale of all or substantially all of the Corporation's properties and assets to any other corporation, the Corporation shall take such action as may be necessary to enable the Optionee to receive upon any subsequent exercise of such option, in whole or in part, in lieu of shares of Common Stock, securities or other assets as were issuable or payable upon such reorganization, consolidation, merger or sale in respect of, or in exchange for such shares of Common Stock.

        8.    Rights of Stockholders.    Neither the Optionee, his or her legal representative, nor other persons entitled to exercise the option shall be or have any rights of a stockholder in the Company in respect of the shares issuable upon exercise of the option granted hereunder, unless and until certificates representing such shares shall have been delivered pursuant to the terms hereof.

        9.    Rights of Director.    Nothing contained in this Agreement shall confer upon Optionee any right to continue to remain as a director of the Corporation.

        10.    Stock Reserved.    The Company shall at all times during the term of this Agreement reserve and keep available such number of shares of its Common Stock as will be sufficient to satisfy the terms of this Agreement.

        11.    Binding Effect.    This Agreement shall be binding upon and inure to the benefit of any successor or successors of the Company.

        IN WITNESS WHEREOF, the parties hereto have caused this Agreement to be executed as of the day and year first above written.

WESTERN GAS RESOURCES, INC.
By:	/s/  PETER A. DEA
	/s/  W. L. STONEHOCKER
Optionee

3

QuickLinks

Exhibit 3

EX-4

QuickLinks -- Click here to rapidly navigate through this document

Exhibit 4

WESTERN GAS RESOURCES, INC.
2002 NON-EMPLOYEE DIRECTORS' STOCK OPTION AGREEMENT

        THIS AGREEMENT, made as of May 19, 2003, by and between Western Gas Resources, Inc. (hereinafter called the "Corporation"), a Delaware corporation, and Walt Stonehocker, a non-employee director of the Corporation (hereinafter called the "Optionee").

RECITALS:

        A.    The Optionee is eligible as a non-employee director of the Corporation to participate in the Western Gas Resources, Inc. 2002 Non-Employee Director's Stock Option Plan (the "Plan").

        B.    The Board of Directors of the Corporation considers it desirable and in the Corporation's best interests that the Optionee be given an opportunity to purchase shares of its Common Stock in furtherance of the Plan to provide incentive for the Optionee to remain as a director of the Company and to promote the success of the Corporation.

        NOW THEREFORE, in consideration of the premises, it is agreed as follows:

        1.    Grant of Option.    The Corporation hereby grants as of May 19, 2003 (the "Grant Date") to the Optionee the right, privilege and option to purchase 2,000 shares of the Common Stock par value $0.10 (the "Common Stock") of the Corporation, at a purchase price of Thirty Eight Dollars and Seventy Six 100ths ($38.76) per share in the manner and subject to the conditions hereafter provided. Said purchase price is not less than the Fair Market Value (as that term is defined in the Plan) of the shares of Common Stock of the Corporation at the time this option was granted.

        2.    Period of Exercise of Option.    This Option may be exercised in whole or in part, or in installments, from time to time, with respect to the shares covered hereby, in the amounts and at the times specified below. The Option or any portion thereof, once it becomes exercisable as specified below, shall remain exercisable until it shall expire in accordance with the provisions of this Agreement.

        (a)   Notwithstanding anything herein to the contrary, no Option or portion thereof granted under this Agreement may be exercised after the earlier of (i) five (5) years after the date the Optionee has the right to exercise such Option or portion thereof, in accordance with paragraph 2(b) below; or (ii) ten (10) years after the Option is granted.

        (b)   Except as expressly provided in Section 2(e), below, an Optionee shall become entitled to exercise that portion of the Option and to purchase the percentage of the Common stock subject to the Option in accordance with the following schedule:

        (1)   Commencing one (1) year from the Grant Date, the Optionee shall have the right to exercise thirty-three and one-third percent (33-1.3%) of the Option and to purchase an additional thirty-three and one-third percent (33¹/3%) subject to the Option.

        (2)   Commencing two (2) years from the Grant Date, the Optionee shall have the right to exercise thirty-three and one-third percent (33-1.3%) of the Option and to purchase an additional thirty-three and one-third percent (33¹/3%) subject to the Option.

        (3)   Commencing three (3) years from the Grant Date, the Optionee shall have the right to exercise thirty-three and one-third percent (33-1.3%) of the Option and to purchase an additional thirty-three and one-third percent (33¹/3%) subject to the Option.

        The Optionee's right to purchase Shares subject to the Option shall be cumulative, so that three (3) years from the Grant Date, the Optionee shall be entitled to exercise one hundred percent (100%) of the Option and to purchase all of the Common Stock subject to the Option, subject to all of the provisions of this Agreements.

        (c)   Except as provided in Sections 2(d) and 2(e), an Optionee may exercise an Option only if, at the time such Option is exercised, such Optionee is a director of, and has continuously since the grant of the Option, been a director of the Corporation or any subsidiary, parent, or predecessor of the Corporation.

        (d)   If an Optionee ceases to be a director for any reason other than (i) his or her death or disability; or (ii) his or her discharge for dishonesty or commission of a crime, the Optionee may, within three (3) months thereafter, and subject to provisions of Sections 2(a), (b) and (c), exercise the Option to the extent that the Option was exercisable as of the date of the Optionee ceased to be a director. All unexercised Options, or portions thereof, shall terminate, be forfeited, and shall lapse upon expiration of said three (3) month period, or immediately if the Optionee ceases to be a director of the Corporation for any of the reasons set forth in (ii), above.

        (e)   If an Optionee dies or becomes disabled while he is a director of the Corporation or ceases to be a director as a result of disability, all of the Options granted to such employee shall become one hundred percent (100%) exercisable, without regard to the provisions of Section 2(b), above. In such event, the Options may be exercised by the disabled director, or the person or persons to whom his or her rights under the Option shall pass by will, or by the applicable laws of descent and distribution; provided, however, that no such Option may be exercised after 180 days from such directors' s date of death, or the date Optionee ceases to be a director as a result of disability, whichever is applicable. Upon expiration of said period, all unexercised Options, or portions thereof, shall terminate, be forfeited, and shall lapse.

        (a)   (f) Notwithstanding the provision of Section (b), above, in the event (1) there is a "Change of Control" of the Corporation; and (2) the Optionee is removed as a director of the Corporation without cause, then all of the options granted to such Optionee under this Agreement shall become one hundred percent (100%) exercisable, subject to the other provisions of this Section 2. For these purposes, a Change of Control of the Corporation shall mean (i) the acquisition by any person or persons acting in concert (including corporations, partnerships, associations or unincorporated organizations), of legal ownership or beneficial ownership (within the meaning of Rule 13d-3, promulgated by the Securities and Exchange Commission and now in effect under the Securities Exchange Act of 1934 (as amended), of a number of voting shares of capital stock of the Corporation greater than either 30% or the number of voting shares of capital stock of the Corporation that are then owned, beneficially (as defined above), by Brion G. Wise, Bill M. Sanderson, Walter L. Stonehocker, Dean Phillips, Ward Sauvage, their immediate families and the companies through which they and their immediate families hold ownership in the Corporation ("the Founders"), whichever is higher; (ii) a merger or consolidation of the Corporation or any of its subsidiaries other than a merger or consolidation immediately following which the directors of the Corporation prior thereto constitute a majority of the of the board of the surviving company or parent thereof; (iii) a change in the majority of the Board pursuant to an actual or threatened proxy contest; or (iv) a sale of substantially all of the Corporation's assets.

        3.    Method of Exercise.    To exercise an Option, the Optionee, or his or her successors, shall give written notice to the Treasurer of the Corporation, at the Corporation's principal office, accompanied by full payment of the Common Stock being purchased. If the Option is exercised by the successor of the Optionee, following his or her death, proof shall be submitted, satisfactory to the Board, of the right of the successor to exercise the Option. The Corporation shall not be required to transfer or deliver any certificate or certificates for shares purchased upon any such exercise of said option: (a) until after compliance with all then applicable requirements of law; and (b) prior to admission of such shares to listing on any stock exchange on which the stock may then be listed. In no event shall the Corporation be required to issue fractional shares to the Optionee.

2

        4.    Limitation Upon Exercise.    The option is not transferable by the Optionee otherwise than by will or the laws of descent and distribution and is exercisable, during the lifetime of Optionee, only by the Optionee.

        5.    Limitation Upon Transfer.    Except as otherwise provided hereto, the option and all rights granted hereunder shall not be transferred by the Optionee, and may not be assigned, pledged, or hypothecated in any way and shall not be subject to execution, attachment or similar process. Upon any attempt to transfer the option, or to assign, pledge, hypothecate or otherwise dispose of such Option or of any rights granted hereunder, contrary to the provisions hereof, or upon the levy of any attachment or similar process upon such option or such rights, such option and such rights shall immediately become null and void.

        6.    Stock Adjustment.    In the event of any change in Common Stock of the Corporation, by reason of a stock split, stock dividend, recapitalization, exchange of shares, or other transaction, the number of shares remaining subject to the option and the option price per share shall be appropriately adjusted by the Board of Directors.

        7.    Corporate Reorganization.    If there shall be any capital reorganization or consolidation or merger of the Corporation with another corporation or corporations, or any sale of all or substantially all of the Corporation's properties and assets to any other corporation, the Corporation shall take such action as may be necessary to enable the Optionee to receive upon any subsequent exercise of such option, in whole or in part, in lieu of shares of Common Stock, securities or other assets as were issuable or payable upon such reorganization, consolidation, merger or sale in respect of, or in exchange for such shares of Common Stock.

        8.    Rights of Stockholders.    Neither the Optionee, his or her legal representative, nor other persons entitled to exercise the option shall be or have any rights of a stockholder in the Company in respect of the shares issuable upon exercise of the option granted hereunder, unless and until certificates representing such shares shall have been delivered pursuant to the terms hereof.

        9.    Rights of Director.    Nothing contained in this Agreement shall confer upon Optionee any right to continue to remain as a director of the Corporation.

        10.    Stock Reserved.    The Company shall at all times during the term of this Agreement reserve and keep available such number of shares of its Common Stock as will be sufficient to satisfy the terms of this Agreement.

        11.    Binding Effect.    This Agreement shall be binding upon and inure to the benefit of any successor or successors of the Company.

        IN WITNESS WHEREOF, the parties hereto have caused this Agreement to be executed as of the day and year first above written.

WESTERN GAS RESOURCES, INC.

By:

Optionee

3

QuickLinks

Exhibit 4