Exhibit d.4

FORM OF SUBSCRIPTION AGENT AGREEMENT

SUBSCRIPTION AGENT AGREEMENT

BY AND BETWEEN

HERCULES TECHNOLOGY GROWTH CAPITAL, INC.

and

[            ]

This Subscription Rights Agreement (the “Agreement”) is made as of                          between Hercules Technology Growth Capital, Inc., a Maryland corporation (the “Company”), and                          (the “Subscription Agent”), and relates to the base prospectus included in the Registration Statement on Form N-2, File No. 333-                , filed by the Company with the Securities and Exchange Commission on                         , 2011, as amended by any amendment filed with respect thereto (the “Registration Statement”) and any supplemental prospectus (together with the base prospectus, the “Prospectus”). Capitalized terms not otherwise defined herein shall have the meanings given to them in the Subscription Certificate (as defined below).

WHEREAS, the Company proposes to make a subscription offer by issuing subscription certificates in the form designated by the Company (“Subscription Certificates”) to stockholders of record (the “Record Date Stockholders”) of its common stock, par value $0.001 per share (“Common Stock”), as of a record date specified by the Company (the “Record Date”) in the Prospectus, pursuant to which each Record Date Stockholder, or any transferee of a Record Date Stockholder (such Record Date Stockholders and transferees thereof, the “Rights Holders”) shall have certain rights (the “Rights”) to subscribe for shares of Common Stock, as described in and upon such terms as are set forth in the Prospectus, a final copy of which has been or, upon availability shall promptly be, delivered to the Subscription Agent; and

WHEREAS, the Company wishes the Subscription Agent to perform certain acts on behalf of the Company, and the Subscription Agent is willing to so act, in connection with the distribution of the Subscription Certificates and the issuance and exercise of the Rights, all upon the terms and conditions set forth herein.

NOW, THEREFORE, in consideration of the foregoing and of the mutual agreements set forth herein, each of the parties agrees as follows:

The Company hereby appoints the Subscription Agent to act as subscription and distribution agent in connection with the distribution of Subscription Certificates and the issuance and exercise of the Rights (the “Offering”) in accordance with the terms set forth in the Agreement, and the Subscription Agent hereby accepts such appointment.

Each Subscription Certificate shall be irrevocable and transferable upon the terms and conditions set forth in the Prospectus. The Subscription Agent shall maintain a register of Subscription Certificates and of the Rights Holders.

If any shares of Common Stock available for purchase pursuant to the Rights offering are not subscribed for by Rights Holders pursuant to the Basic Subscription, the Subscription Agent shall allot such shares to Rights Holders who have properly subscribed for such shares pursuant to an over-subscription privilege on the terms and subject to the conditions set forth in the Prospectus, including as to proration. In addition, any Rights Holder other than a Record Date Stockholder who exercises Rights is entitled to subscribe for any Remaining Shares that are not otherwise subscribed for by Record Date Stockholders pursuant to their over-subscription privilege, on the terms and subject to the conditions set forth in the Prospectus, including as to proration. We refer to these over-subscription privileges as the “Over-Subscription Privilege.”]

Irregular, incomplete or unpaid subscriptions not otherwise covered by specific instructions herein shall be submitted to an appropriate officer of the Company and handled in accordance with his or her instructions. Such instructions shall be reasonably documented by the Subscription Agent indicating, among other things, the instructing officer and the date thereof.

The Subscription Agent shall deliver Statements of Holding reflecting new shares of Common Stock purchased pursuant to the Basic Subscription [and, if applicable, those shares purchased pursuant to the exercise of the Over-Subscription Privilege] as soon as practicable after the Expiration Date, after all allocations have been effected and full payment for such shares has been received and cleared.

Daily, during the period commencing on the date of mailing the Subscription Certificates until and including the Confirmation Date, the Subscription Agent shall report by telephone, facsimile or electronic mail to a designated representative(s) of the Company, as instructed by such designated representative, the following information:

The Subscription Agent shall report by telephone, facsimile or electronic mail, as instructed by such designated representative, not later than 5:00 p.m., New York City time, on the first business day following the Expiration Date, (i) the total number of Rights exercised by each Rights Holder pursuant to the Basic Subscription [and the Over-Subscription Privilege and shares of Common Stock related thereto], (ii) the total number of Rights verified to be in proper form for exercise, rejected for exercise and being processed, and all payments received in connection therewith, (iii) with respect to the Dealer Managers and each soliciting dealer, the number of Rights exercised on forms indicating the Dealer Manager or such soliciting dealer, as the case may be, as the broker-dealer with respect to such exercise, and (iv) any such other information as may be reasonably requested by the Company or such designated representative.

In addition, the Subscription Agent shall perform the services set forth in Exhibit A.

If any Subscription Certificate is lost, stolen, mutilated or destroyed, the Subscription Agent may, on such terms as will serve to indemnify and protect the Company and the Subscription Agent as the Subscription Agent may in its reasonable discretion impose on the relevant Rights Holder (which shall, in the case of a mutilated Subscription Certificate, include the surrender and cancellation thereof), issue a new Subscription Certificate to such Rights Holder of like denomination in substitution for the Subscription Certificate so lost, stolen, mutilated or destroyed.

The Company agrees that it shall pay to the Subscription Agent compensation for their respective services hereunder in accordance with the Fee and Service Schedule attached hereto as Exhibit B. The Company further agrees that it shall reimburse the Subscription Agent for its reasonable, documented out-of-pocket expenses incurred in the performance of its respective duties as such; provided, however, that the Company shall not be required to reimburse the Subscription Agent for any such expenses incurred which exceed $                 in the aggregate without its prior written consent.

The Subscription Agent undertakes the respective duties and obligations imposed by this Agreement upon the following terms and conditions:

Promptly after the receipt by the Subscription Agent of notice of any Claim, the Subscription Agent shall notify the Company thereof in writing. In no case shall the Company be liable under this Section 12 to the extent it is materially prejudiced by failure of any indemnified party to deliver prompt notice of any Claim. The Company shall be entitled to participate at its own expense in the defense of any Claim, and, if it so elects at any time after receipt of such notice, it may assume the defense of any suit brought to enforce any Claim. For the purposes of this Section 12, the term “liability and expense” shall include all reasonable costs and expenses, including, but not limited to, reasonable counsel fees (including one local counsel, if necessary) and disbursements, paid or incurred in investigating or defending against any such claim, demand, action, suit, proceeding or investigation.

This Agreement does not constitute an agreement for a partnership or joint venture among the parties. The Subscription Agent shall act hereunder as agent of the Company solely to the limited extent set forth in this Agreement, but shall not assume any fiduciary duties to, or have any rights, power or authority on behalf of, the Company or any of its affiliates, equity holders or creditors or of any other person or entity not expressly set forth in this Agreement. Any duties of the Subscription Agent arising out of its engagements pursuant to this Agreement shall be owed solely to the Company. No party shall make any commitments with third parties that are binding on any other party without such other party’s prior written consent, and none of the Subscription Agent, employees, or

representatives or contractors of the Subscription Agent shall be deemed to be employees of the Company or any of its affiliates.

Except as explicitly stated elsewhere in this Agreement, nothing under this Agreement is intended or shall be construed to give any rights, benefits, remedies or claims under or by reason of this Agreement or any part thereof to anyone other than the Subscription Agent and the Company, and the duties and responsibilities undertaken pursuant to this Agreement shall be for the sole and exclusive benefit of the Subscription Agent and the Company.

In the event any party is unable to perform its obligations under the terms of this Agreement because of acts of God, strikes, terrorist acts, equipment or transmission failure or damage reasonably beyond its control or other cause reasonably beyond its control, such party shall not be liable for damages to any other party resulting from such failure to perform or otherwise from such causes. Performance under this Agreement shall resume when the affected party or parties are able to perform substantially its or their duties.

No party to this Agreement shall be liable to any other party for any consequential, indirect, penal, special or incidental damages under any provisions of this Agreement or for any consequential, indirect, penal, special or incidental damages arising out of any act or failure to act hereunder even if that party has been advised of or has foreseen the possibility of such damages.

If any provision of this Agreement shall be held invalid, unlawful or unenforceable, the validity, legality and enforceability of the remaining provisions shall not in any way be affected or impaired.

The captions and descriptive headings herein are for the convenience of the parties only. They do not in any way modify, amplify, alter or give full notice of the provisions hereof.

The Agreement shall remain in effect until the earlier of (i) thirty (30) days after the Expiration Date; (ii) it is terminated by the Company, on the one hand, or the Subscription Agent with respect to the service provided by such Subscription Agent, on the other, upon a material breach of this Agreement by the other which remains uncured for fifteen (15) days after written notice of such breach has been provided to such other party; or (iii) thirty (30) days’ written notice has been provided by the Company, on the one hand, or the Subscription Agent, on the other. Upon termination of the Agreement, the Subscription Agent shall retain a copy of all canceled Subscription Certificates and related documentation as required by applicable law; all documentation and information related to the services performed under the Agreement shall be promptly delivered to the Company.

Until further notice in writing by any party hereto to the other parties, all written reports, notices and other communications between any Subscription Agent, on the one hand, and the Company, on the other, required or permitted hereunder shall be delivered or mailed by first class mail, postage prepaid, facsimile or overnight courier guaranteeing next day delivery, addressed as follows:

If to the Company, to:

Hercules Technology Growth Capital, Inc.

400 Hamilton Avenue, Suite 310

Palo Alto, CA 94301

Attn: Chief Financial Officer

With a copy (which shall not constitute notice) to:

Sutherland Asbill & Brennan LLP

1275 Pennsylvania Avenue, NW

Washington, DC 20004

Attn: Cynthia M. Krus

If to the Subscription Agent, to:

The provisions of Paragraphs 8, 12, 19, 24-28 shall survive any termination, for any reason, of this Agreement.

This Agreement constitutes the entire agreement between the parties hereto with respect to the transactions contemplated hereby and supersedes any prior agreement with respect to the subject matter hereof, whether oral or written.

No term or provision of this Agreement may be amended, changed, altered or modified except by written instrument executed by the each of the parties to this Agreement.

The Subscription Agent may, without the consent or concurrence of the Rights Holders in whose names Subscription Certificates are registered but with the prior written consent of the Company, by supplemental agreement or otherwise, make any change or correction in a Subscription Certificate that it shall have been advised by counsel (who may be counsel for the Company) is appropriate to cure any ambiguity or to correct any defective or inconsistent provision or clerical omission or mistake or manifest error therein or herein contained, and which shall not be inconsistent with the provisions of the Subscription Certificate except insofar as any such change may confer additional rights upon the Rights Holder.

At any time, each of the parties hereto may, to the extent legally allowed, (i) extend the time for, or waive, the performance of any of the covenants, obligations or agreements of the other parties hereto, (ii) waive any inaccuracies or breaches in the representations and warranties contained herein or in any certificate delivered by another party pursuant hereto or (iii) waive compliance with any of the conditions of another party contained in this Agreement. Any agreement on the part of any of the parties hereto to any such extension or waiver shall be valid only if set forth in a written instrument signed by the party or parties against whom such extension or waiver is to be enforced. Any waiver of any term or condition shall not be construed as a waiver of any subsequent breach or a subsequent waiver of the same term or condition, or as a waiver of any other term or condition, of this Agreement. The failure of any of the parties hereto to assert any of its rights under this Agreement shall not constitute a waiver of such rights or any other rights.

Each of the parties hereto: (i) irrevocably and unconditionally submits to the exclusive jurisdiction of the U.S. District Court for the Southern District of New York or, if such court does not have jurisdiction, the New York State Supreme Court in the Borough of Manhattan, The City of New York, in any action arising out of or relating to this Agreement, (ii) agrees that all claims in respect of the action may be heard and determined in any such court and (iii) agrees not to bring any action arising out of or relating to this Agreement in any other court.

In any such action, each of the parties hereto irrevocably and unconditionally waives and agrees not to assert by way of motion, as a defense or otherwise, any claims that it is not subject to the jurisdiction of the above court, that such action is brought in an inconvenient forum or that the venue of such action is improper. Each of the parties hereto also agrees that any final and nonappealable judgment against a party hereto in connection with any action shall be conclusive and binding on such party and that such award or judgment may be enforced in any court of competent jurisdiction, either within or outside of the United States. A certified or exemplified copy of such award or judgment shall be conclusive evidence of the fact and amount of such award or judgment.

Without limiting the foregoing, each party hereto agrees that service of process on such party at the address provided in Section 24 shall be deemed effective service of process on such party.

Each of the parties hereto irrevocably waives any and all right to trial by jury in any action, proceeding or counterclaim (whether based upon contract, tort or otherwise) in any way arising out of or relating to this Agreement.

This Agreement shall be governed by and construed in accordance with the laws of the State of New York.

This Agreement may be executed in one or more counterparts (including by facsimile or electronic transmission), each of which shall be deemed an original and all of which together shall be considered one and the same agreement.

IN WITNESS WHEREOF, the parties hereto have caused this Agreement to be executed by their respective officers, hereunto duly authorized, as of the day and year first above written.

EXHIBIT A

ADDITIONAL SERVICES

TO BE PERFORMED BY SUBSCRIPTION AGENT