EX-(a)(1)(iv)

THE OFFER AND WITHDRAWAL RIGHTS EXPIRE AT 12:01 AM

(NEW YORK CITY TIME) ON MAY 7, 2015, UNLESS THE OFFER IS EXTENDED.

We have been engaged by Ghrian Acquisition Inc., a Delaware corporation (“Purchaser”) and a wholly owned subsidiary of Horizon Pharma, Inc., a Delaware corporation (“Parent”) and a wholly owned subsidiary of Horizon Pharma plc, a public limited company organized under the laws of Ireland, to act as information agent in connection with Purchaser’s offer to purchase all of the shares of common stock, par value $0.0001 per share, including any associated rights to purchase capital stock (such rights, together with such shares of common stock, the “Shares”), of Hyperion Therapeutics, Inc., a Delaware corporation (“Hyperion”), that are issued and outstanding at a price of $46.00 per Share, net to the seller in cash (the “Offer Price”), without interest, less any applicable withholding taxes, upon the terms and subject to the conditions set forth in the Offer to Purchase, dated April 9, 2015 (as it may be amended or supplemented from time to time, the “Offer to Purchase”), and the related Letter of Transmittal (as it may be amended or supplemented from time to time, the “Letter of Transmittal” and, together with the Offer to Purchase, the “Offer”) enclosed herewith. Please furnish copies of the enclosed materials to those of your clients for whom you hold Shares registered in your name or in the name of your nominee.

THE BOARD OF DIRECTORS OF HYPERION UNANIMOUSLY RECOMMENDS THAT

STOCKHOLDERS TENDER ALL OF THEIR SHARES PURSUANT TO THE OFFER.

The conditions of the Offer are described in Section 15 of the Offer to Purchase.

For your information and for forwarding to your clients for whom you hold Shares registered in your name or in the name of your nominee, we are enclosing the following documents:

2. The Letter of Transmittal for your use in accepting the Offer and tendering Shares and for the information of your clients, together with the included IRS Form W-9;

3. A notice of guaranteed delivery to be used to accept the Offer if Shares and all other required documents are not immediately available or cannot be delivered to Computershare Trust Company, N.A. (the “Depositary”) by 12:01 am (New York City time) on May 7, 2015 (the “Expiration Time,” unless the Offer is extended pursuant to and in accordance with the terms of the Merger Agreement, in which event “Expiration Time” will mean the latest time and date at which the Offer, as so extended, will expire and the date on which the Expiration Time occurs is referred to herein as the “Expiration Date”)) or if the procedure for book-entry transfer cannot be completed by the Expiration Date (the “Notice of Guaranteed Delivery”);

4. A form of letter that may be sent to your clients for whose accounts you hold Shares registered in your name or in the name of your nominee, with space provided for obtaining such clients’ instructions with regard to the Offer;

We urge you to contact your clients as promptly as possible. Please note that the Offer and withdrawal rights will expire at 12:01 am (New York City time) on May 7, 2015, unless the Offer is extended.

The Offer is being made pursuant to the Agreement and Plan of Merger, dated as of March 29, 2015 (as it may be amended or supplemented from time to time in accordance with its terms, the “Merger Agreement”), by and among Parent, Purchaser and Hyperion, pursuant to which, as soon as practicable following the time Purchaser accepts, for the first time, for payment Shares validly tendered and not properly withdrawn pursuant to the Offer (the “Acceptance Time”) and subject to the satisfaction or waiver of the remaining conditions set forth in the Merger Agreement, Purchaser will merge with and into Hyperion (the “Merger”), with Hyperion continuing as the surviving corporation in the Merger and as a wholly owned subsidiary of Parent. At the effective time of the Merger (the “Effective Time”), each Share then outstanding (other than Shares (i) held by Hyperion as treasury stock, (ii) held by Parent, Purchaser or any other wholly owned subsidiary of Parent or (iii) held by a Hyperion stockholder who is entitled to demand and properly demands appraisal for such Shares in accordance with Section 262 of the Delaware General Corporation Law (the “DGCL”), and in the case of (i) and (ii), such Shares will be cancelled and will cease to exist, and no consideration will be delivered in exchange therefor) will be converted into the right to receive the Offer Price, net in cash, without interest (less any required withholding taxes). Under no circumstances will interest be paid with respect to the purchase of Shares pursuant to the Offer, regardless of any extension of the Offer or any delay in making payment for Shares.

After careful consideration, the board of directors of Hyperion has unanimously (i) determined that the Merger Agreement and the transactions contemplated by the Merger Agreement, including the Offer and the Merger, are advisable, fair to and in the best interests of Hyperion and its stockholders; (ii) approved the execution, delivery and performance by Hyperion of the Merger Agreement and the consummation of the transactions, including the Offer and the Merger and declared advisable the Merger Agreement and the transactions contemplated hereby and thereby, including the Offer and the Merger on the terms and subject to the conditions set forth therein; (iii) agreed that the Merger Agreement would be subject to Section 251(h) of the DGCL and (iv) resolved to recommend that Hyperion’s stockholders accept the Offer, tender their Shares to Purchaser pursuant to the Offer and, to the extent applicable, adopt and approve the Merger Agreement and approve the Merger.

For Shares to be properly tendered pursuant to the Offer, (a) the share certificates or confirmation of receipt of such Shares under the procedure for book-entry transfer, together with a properly completed and duly executed Letter of Transmittal, including any required signature guarantees, or, in the case of book-entry transfers, either such Letter of Transmittal or an Agent’s Message (as defined in Section 3 of the Offer to Purchase) in lieu of such Letter of Transmittal, and any other documents required in the Letter of Transmittal, must be timely received by the Depositary or (b) the tendering stockholder must comply with the guaranteed delivery procedures, all in accordance with the Offer to Purchase and the Letter of Transmittal. You may gain some additional time by making use of the Notice of Guaranteed Delivery.

Except as set forth in the Offer to Purchase, Purchaser will not pay any fees or commissions to any broker or dealer or other person for soliciting tenders of Shares pursuant to the Offer. Purchaser will, however, upon request, reimburse brokers, dealers, commercial banks, trust companies and other nominees for customary mailing and handling expenses incurred by them in forwarding the offering material to their customers. Purchaser will pay all stock transfer taxes applicable to its purchase of Shares pursuant to the Offer, subject to Instruction 6 of the Letter of Transmittal.

You may contact us as Information Agent with questions and requests for assistance, and for additional copies of the Offer to Purchase, the Letter of Transmittal, the Notice of Guaranteed Delivery and other tender offer materials, at the address and telephone number set forth on the back cover of the Offer to Purchase.

Nothing contained herein or in the enclosed documents shall render you the agent of Purchaser, Parent, Hyperion, the Information Agent, or the Depositary or any affiliate of any of them or authorize you or any other person to use any document or make any statement on behalf of any of them in connection with the Offer other than the enclosed documents and the statements contained therein.