COWEN AND COMPANY, LLC PIPER SANDLER & CO. As Representatives of the several Underwriters c/o Cowen and Company, LLC New York, New York 10022

Exhibit 99.4

November 27, 2022

XXXXX AND COMPANY, LLC

XXXXX XXXXXXX & CO.

    As Representatives of the several Underwriters

c/o Cowen and Company, LLC

000 Xxxxxxxxx Xxxxxx

Xxx Xxxx, Xxx Xxxx 00000

Re: Aspen Aerogels, Inc. – Registration Statement on Form S-3 for Shares of Common Stock

Dear Sirs and Madams:

This letter agreement (the “Agreement”) is being delivered to you in connection with the proposed Underwriting Agreement (the “Underwriting Agreement”) between Aspen Aerogels, Inc., a Delaware corporation (the “Company”) and Xxxxx and Company, LLC (“Cowen”) and Xxxxx Xxxxxxx & Co., as representatives ( “Piper” and together with Cowen, the “Representatives”) of a group of underwriters (collectively, the “Underwriters”), to be named therein, and the other parties thereto (if any), relating to the proposed public offering of shares of common stock, par value $0.00001 per share (the “Common Stock”) of the Company (the “Public Offering”).

In order to induce the Underwriters to enter into the Underwriting Agreement, and in light of the benefits that the offering of the Common Stock will confer upon the undersigned in his, her or its capacity as a securityholder and/or an officer, director or employee of the Company, and for good and valuable consideration, the receipt and sufficiency of which are hereby acknowledged, the undersigned agrees with each Underwriter that, during the period beginning on the date hereof through and including the date that is the 60^th day after the date of the Underwriting Agreement (the “Lock-Up Period”), the undersigned will not, and will not cause or direct any of its affiliates to, without the prior written consent of Cowen and Xxxxx, directly or indirectly, (i) offer, sell, assign, transfer, pledge, contract to sell, lend or otherwise dispose of, or announce the intention to otherwise dispose of, any shares of Common Stock (including, without limitation, Common Stock which may be deemed to be beneficially owned by the undersigned in accordance with the rules and regulations promulgated under the Securities Act of 1933, as amended (the “Securities Act”) as the same may be amended or supplemented from time to time (such shares, the “Beneficially Owned Shares”)) or securities convertible into or exercisable or exchangeable for Common Stock, (ii) enter into, or announce the intention to enter into, any swap, hedge or similar agreement or arrangement (including, without limitation, the purchase or sale of, or entry into, any put or call option, or combination thereof, forward, swap or any other derivative transaction or instrument, however described or defined) that transfers, is designed to transfer or reasonably could be expected to transfer (whether by the undersigned or someone other than the undersigned) in whole or in part, directly or indirectly, the economic risk of ownership of the Beneficially Owned Shares or securities convertible into or exercisable or exchangeable for Common Stock, whether now owned or hereafter acquired by the undersigned or with respect to which the undersigned has or hereafter acquires the power of disposition (the “Prohibited Activity”), or (iii) engage in, or announce the intention to engage in, any short selling of the Common Stock or securities convertible into or exercisable or exchangeable for Common Stock. The undersigned represents and warrants that the undersigned is not, and has not caused or directed any of its affiliates to be or become, currently a party to any agreement or arrangement that is designed to or which reasonably could be expected to lead to or result in any Prohibited Activity during the Lock-Up Period.

The restrictions set forth in the immediately preceding paragraph shall not apply to:

(1) if the undersigned is a natural person, any transfers made by the undersigned (a) as a bona fide gift to any member of the immediate family (as defined below) of the undersigned or to a trust the beneficiaries of which are exclusively the undersigned or members of the undersigned’s immediate family, (b) by will or intestate succession upon the death of the undersigned or (c) as a bona fide gift to a charity or educational institution,

(2) if the undersigned is a corporation, partnership, limited liability company or other business entity, any transfers to any stockholder, partner or member of, or owner of a similar equity interest in, the undersigned, as the case may be, if, in any such case, such transfer is not for value,

(3) if the undersigned is a corporation, partnership, limited liability company or other business entity, any transfer made by the undersigned (a) in connection with the sale or other bona fide transfer in a single transaction of all or substantially all of the undersigned’s capital stock, partnership interests, membership interests or other similar equity interests, as the case may be, or all or substantially all of the undersigned’s assets, in any such case not undertaken for the purpose of avoiding the restrictions imposed by this Agreement or (b) to another corporation, partnership, limited liability company or other business entity so long as the transferee is an affiliate (as defined below) of the undersigned and such transfer is not for value,

(4) transactions relating to shares of Common Stock or other securities acquired in the open market after the completion of the offering,

(5) any transfers pursuant to a court order or settlement agreement related to the distribution of assets in connection with the dissolution of a marriage or civil union, provided, that (1) no public disclosure or filing under the Securities Exchange Act of 1934, as amended (the “Exchange Act”) shall be voluntarily made during the Lock-Up Period and (2) any required filing under the Exchange Act made during the Lock-Up Period shall clearly indicate in the footnotes thereto that the filing relates to the circumstances described in this clause, unless otherwise prohibited by such court order or settlement agreement,

(6) the establishment of any contract, instruction or plan that satisfies all of the requirements of Rule 10b5-1 (a “Rule 10b5-1 Plan”) under the Exchange Act; provided, however, that no sales of Common Stock or securities convertible into, or exchangeable or exercisable for, Common Stock, shall be made pursuant to a Rule 10b5-1 Plan prior to the expiration of the Lock-Up Period (as the same may be extended pursuant to the provisions hereof); provided further, that the Company is not required to report the establishment of such Rule 10b5-1 Plan in any public report or filing with the Securities and Exchange Commission under the Exchange Act during the Lock-Up Period and does not otherwise voluntarily effect any such public filing or report regarding such Rule 10b5-1 Plan,

(7) any transfers or commitments to transfer pursuant to a merger, consolidation, tender offer or other similar transaction that are approved by the Board of Directors of the Company involving a Change of Control (as defined below) or reverse merger, provided, that in the event that such merger, consolidation, tender offer or other such transaction or reverse merger is not completed, such shares of Common Stock or other securities held by the undersigned shall remain subject to the provisions of this Lock-Up Letter Agreement, and

(8) the transfer of shares of Common Stock or securities convertible into, or exercisable or exchangeable for, shares of Common Stock to the Company in connection with the termination of the undersigned’s employment with the Company, provided, that (1) no public disclosure or filing under the Exchange Act shall be voluntarily made during the Lock-Up Period and (2) any required filing under the Exchange Act made during the Lock-Up Period shall clearly indicate in the footnotes thereto that the filing relates to the circumstances described in this clause.

provided, however, that in the case of any transfer described in clause (1), (2) or (3) above, it shall be a condition to the transfer that (A) the transferee executes and delivers to Cowen, acting on behalf of the Underwriters, not later than one business day prior to such transfer, a written agreement, in substantially the form of this Agreement (it being understood that any references to “immediate family” in the agreement executed by such transferee shall expressly refer only to the immediate family of the undersigned and not to the immediate family of the transferee) and otherwise satisfactory in form and substance to Cowen and Xxxxx, and (B) in the case of any transfer described in clause (1), (2) or (3) above, no public announcement or filing is voluntarily made regarding such transfer during the Lock-Up Period and if the undersigned is required to file a report under Section 16(a) of the Exchange Act, reporting a reduction in beneficial ownership of shares of Common Stock or Beneficially Owned Shares or any securities convertible into or exercisable or exchangeable for Common Stock or Beneficially Owned Shares during the Lock-Up Period, the undersigned shall include a statement in such report to the effect that, (A) in the case of any transfer pursuant to clause (1) above, such transfer is being made as a gift or by will or intestate succession, (B) in the case of any transfer pursuant to clause (2) above, such transfer is being made to a stockholder, partner or member of, or owner of a similar equity interest in, the undersigned and is not a transfer for value, (C) in the case of any transfer pursuant to clause (3) above, such transfer is being made either (a) in connection with the sale or other bona fide transfer in a single transaction of all or substantially all of the undersigned’s capital stock, partnership interests, membership interests or other similar equity interests, as the case may be, or all or substantially all of the undersigned’s assets or (b) to another corporation, partnership, limited liability company or other business entity that is an affiliate of the undersigned and such transfer is not for value. For purposes of this paragraph, “immediate family” shall mean a spouse, child, grandchild or other lineal descendant (including by adoption), father, mother, brother or sister of the undersigned; and “affiliate” shall have the meaning set forth in Rule 405 under the Securities Act.

For purposes of this Lock-Up Letter Agreement, “Change of Control” shall mean the consummation of any bona fide third-party tender offer, merger, consolidation or other similar transaction, in one transaction or a series of related transactions, the result of which is that any “person” (as defined in Section 13(d)(3) of the Exchange Act), or group of persons, becomes the beneficial owner (as defined in Rules 13d-3 and 13d-5 of the Exchange Act) of more than 50% of the voting capital stock of the Company (or the surviving entity).

For avoidance of doubt, nothing in this Agreement prohibits the undersigned from exercising any options or warrants to purchase Common Stock (which exercises may be effected on a cashless basis to the extent the instruments representing such options or warrants permit exercises on a cashless basis) or converting any convertible security outstanding on the date hereof, it being understood that any Common Stock issued upon such exercises or conversions will be subject to the restrictions of this Agreement and provided, however, that no public announcement or filing is voluntarily made regarding such exercise or conversion during the Lock-Up Period and provided that if the undersigned is required to file a report under Section 16(a) of the Exchange Act reporting a reduction in beneficial ownership of such options or warrants or convertible securities during the Lock-Up Period, the undersigned shall include a statement in such report to the effect that the disposition relates to the exercise of an option or warrant, or conversion of a convertible security, as applicable, and that the shares of Common Stock received upon exercise or conversion are subject to the restrictions of this Agreement.

In order to enable this covenant to be enforced, the undersigned hereby consents to the placing of legends or stop transfer instructions with the Company’s transfer agent with respect to any Common Stock or securities convertible into or exercisable or exchangeable for Common Stock.

The undersigned further agrees that it will not, during the Lock-Up Period, make any demand or request for or exercise any right with respect to the registration under the Securities Act, of any shares of Common Stock or other Beneficially Owned Shares or any securities convertible into or exercisable or exchangeable for Common Stock or other Beneficially Owned Shares.

The undersigned hereby represents and warrants that the undersigned has full power and authority to enter into this Agreement and that this Agreement has been duly authorized (if the undersigned is not a natural person), executed and delivered by the undersigned and is a valid and binding agreement of the undersigned. This Agreement and all authority herein conferred are irrevocable and shall survive the death or incapacity of the undersigned (if a natural person) and shall be binding upon the heirs, personal representatives, successors and assigns of the undersigned.

This Agreement shall be governed by and construed in accordance with the internal laws of the State of New York applicable to agreements made and to be performed in such state.

This Agreement may be delivered via facsimile, electronic mail (including pdf or any electronic signature complying with the U.S. federal ESIGN Act of 2000, e.g., xxx.xxxxxxxx.xxx or xxx.xxxxxxxx.xxx) or other transmission method and any copy so delivered shall be deemed to have been duly and validly delivered and be valid and effective for all purposes.

The undersigned acknowledges and agrees that whether or not any public offering of Common Stock actually occurs depends on a number of factors, including market conditions.

It is understood that (i) if the Company notifies the Underwriters in writing that it does not intend to proceed with the Public Offering, (ii) if the Underwriting Agreement does not become effective on or before December 31, 2022 or (iii) if the Underwriting Agreement (other than the provisions thereof that survive termination) shall terminate or be terminated prior to payment for and delivery of the shares of Common Stock, then the undersigned will be released from the obligations under this Agreement.

[Signature page follows]

Very truly yours,
WOOD RIVER CAPITAL, LLC
By:		/s/ Xxxxxxx Xxx
Name:		Xxxxxxx Xxx
Title:		President
Address: Wood River Capital, LLC
0000 Xxxx 00xx Xxxxxx Xxxxx
Xxxxxxx, Xxxxxx 00000
Attention: President
Email: xxxxxxxxxxxx@xxxxxx.xxx
With a copy (which shall not constitute
actual or constructive notice) to:
Xxxxxxx LLP
0000 Xxxxxx Xxxxxx, Xxxxx 0000
Xxxxxx Xxxx, Xxxxxxxx 00000
Attention: Xxxx Xxxxxxx; Xxxxxxx Xxxxxxxxx
Email: xxxx.xxxxxxx@xxxxxxx.xxx;
xxxxxxx.xxxxxxxxx@xxxxxxx.xxx
[Signature Page to Lock-Up Agreement]