UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE TO/A

(Amendment No. 4)

Tender Offer Statement under Section 14(d)(1) or 13(e)(1)

of the Securities Exchange Act of 1934

AMERICAN PACIFIC CORPORATION

(Names of Subject Company (issuer))

FLAMINGO MERGER SUB CORP.

(Name of Filing Persons (Offeror)) a wholly-owned subsidiary of

FLAMINGO PARENT CORP.

(Name of Filing Persons (Parent of Offeror))

H.I.G. BAYSIDE DEBT & LBO FUND II, L.P.

(Names of Filing Persons (Other Person))

COMMON STOCK, $0.10 PAR VALUE PER SHARE

(Title of Class of Securities)

028740108

(CUSIP Number of Class of Securities)

Flamingo Merger Sub Corp.

Flamingo Parent Corp.

c/o H.I.G. Capital, LLC

1450 Brickell Avenue, 31^st Floor

Miami, Florida 33131

Attention: Fraser Preston

Phone: (305) 379-2322

(Name, address and telephone number of person authorized to receive notices and communications on behalf of filing persons)

with copies to:

Carl P. Marcellino

Ropes & Gray LLP

1211 Avenue of the Americas

New York, NY 10036-8704 

Phone: (212) 841-0623

CALCULATION OF FILING FEE

Transaction valuation*	Amount of filing fee**
$394,055,136	$50,754.30

* Estimated solely for purposes of calculating the filing fee. The transaction valuation was calculated by multiplying the offer price of $46.50 per share of common stock of American Pacific Corporation (“AMPAC”), par value $0.10 per share, (“Shares”) by 8,474,304 Shares, which, based on information provided by AMPAC as of January 21, 2014, is the sum of (i) 7,997,913 Shares outstanding and (ii) 476,391 Shares authorized and reserved for issuance pursuant to outstanding options to purchase Shares.

** The filing fee, calculated in accordance with Rule 0-11 of the Securities Exchange Act of 1934, as amended, and Fee Rate Advisory #1 for fiscal year 2014, issued August 30, 2013, is calculated by multiplying the Transaction Valuation by 0.0001288.

x Check the box if any part of the fee is offset as provided by Rule 0-11(a)(2) and identify the filing with which the offsetting fee was previously paid. Identify the previous filing by registration statement number, or the Form or Schedule and the date of its filing.

Amount Previously Paid: $50,754.30	Filing Party: Flamingo Merger Sub Corp.
Form or Registration No.: Schedule TO-T	Date Filed: January 24, 2014

¨ Check the box if the filing relates solely to preliminary communications made before the commencement of a tender offer.

Check the appropriate boxes below to designate any transactions to which the statement relates:

x third-party tender offer subject to Rule 14d-1.

¨ issuer tender offer subject to Rule 13e-4.

¨ going-private transaction subject to Rule 13e-3.

¨ amendment to Schedule 13D under Rule 13d-2.

Check the following box if the filing is a final amendment reporting the results of the tender offer: ¨

If applicable, check the appropriate box(es) below to designate the appropriate rule provision(s) relied upon:

¨ Rule 13e-4(i) (Cross-Border Issuer Tender Offer)

¨ Rule 14d-1(d) (Cross-Border Third-Party Tender Offer)

This Amendment No. 4 amends and supplements the Tender Offer Statement on Schedule TO filed with the Securities and Exchange Commission (the “SEC”) on January 24, 2014, as amended by Amendment No. 1 filed with the SEC on February 3, 2014, Amendment No. 2 filed with the SEC on February 5, 2014 and Amendment No. 3 filed with the SEC on February 7, 2014 (which, together with any subsequent amendments or supplements thereto, collectively constitutes this “Schedule TO”). This Schedule TO relates to the tender offer by Flamingo Merger Sub Corp., a Delaware corporation (“Purchaser”) and a wholly-owned subsidiary of Flamingo Parent Corp., a Delaware corporation (“Parent”), which is controlled by H.I.G. Bayside Debt & LBO Fund II, a Delaware limited partnership (“Sponsor”), for all of the outstanding shares of common stock, par value $0.10 per share (“Shares”), of American Pacific Corporation, a Delaware corporation (“AMPAC”), at a price of $46.50 per share, net to the seller in cash, without interest thereon and less any applicable withholding taxes, upon the terms and conditions set forth in the offer to purchase dated January 24, 2014 (the “Offer to Purchase”), a copy of which is attached as Exhibit (a)(1)(A) to the Schedule TO, and in the related letter of transmittal (the “Letter of Transmittal”), a copy of which is attached as Exhibit (a)(1)(B) to the Schedule TO, which, together with any other related materials, as each may be amended or supplemented from time to time, collectively constitute the “Offer.”

All the information set forth in the Offer to Purchase is incorporated by reference herein in response to Items 1 through 9 and Item 11 of this Schedule TO, and is supplemented by the information specifically provided in this Schedule TO.

This Amendment No. 4 is being filed to amend and supplement Items 1 through 9 and Item 11 as reflected below.

Items 1 through 9 and Item 11.

Items 1 through 9 and Item 11 of the Schedule TO are hereby amended and supplemented as follows:

On January 24, 2014, H.I.G. and AMPAC filed their respective Premerger Notification and Report Forms with the FTC and the Antitrust Division of the Department of Justice relating to the proposed Offer and Merger. On February 10, 2014 at 11:59 p.m., the waiting period under the HSR Act relating to the Offer expired. Accordingly, the condition of the Offer relating to the expiration of the waiting period under the HSR Act has been satisfied.

The Offer continues to be subject to the other conditions set forth in Section 15—“Certain Conditions of the Offer” of the Offer to Purchase, including the waiting period under the Austrian Competition Laws.

SIGNATURES

After due inquiry and to the best of their knowledge and belief, each of the undersigned certifies that the information set forth in this statement is true, complete and correct.

Dated: February 12, 2014

FLAMINGO MERGER SUB CORP.
By:	/s/ Fraser Preston
Name:	Fraser Preston
Title:	President
FLAMINGO PARENT CORP.
By:	/s/ Fraser Preston
Name:	Fraser Preston
Title:	President
H.I.G. BAYSIDE DEBT & LBO FUND II, L.P. By:   H.I.G. Bayside Advisors, II, LLC          its General Partner By: H.I.G.-GPII, Inc.          its General Partner
By:	/s/ Richard Siegel
Name:	Richard Siegel
Title:	Authorized Signatory