Private Placement Unit Purchase Agreement

Exhibit 10.4

September 30, 2025

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The Woodlands, TX 77380

Ladies and Gentlemen:

AA Mission Acquisition Corp. II (the “Company”), a blank check company formed for the purpose of acquiring one or more businesses or entities (a “Business Combination”), intends to register its securities under the Securities Act of 1933, as amended (the “Securities Act”), in connection with its initial public offering (“IPO”), pursuant to a registration statement on Form S-1 (the “Registration Statement”). The undersigned hereby commits that it will purchase 334,000 units of the Company (“Private Units”) for a purchase price of $3,340,000 (the “Private Unit Purchase Price”), each Private Unit entitles the holder thereof to one Class A ordinary share and one-half of one redeemable warrant (“Private Placement Warrants”) to purchase one Class A ordinary share at $11.50 per share.

The undersigned hereby agrees that it will purchase an additional number of units of the Company (“Over-Allotment Units”), up to a maximum of 26,250 Over-Allotment Units at a purchase price of $10.00 (the “Over-Allotment Purchase Price”), or a maximum of 360,250 Private Units for a total purchase price of $3,602,500 (the Over-Allotment Unit Purchase Price, together with the Private Unit Purchase Price, being the “Purchase Price”), in the event Clear Street LLC (“Clear Street”) exercises its over-allotment option, such that the amount held in the trust account (as described in the Registration Statement) does not fall below $10.00 per unit sold by the Company in the IPO.

At least twenty-four (24) hours prior to the effective date of the Registration Statement, the undersigned will cause the Private Unit Purchase Price to be delivered to Continental Stock Transfer & Trust Company (“Continental”), by wire transfer as set forth in the instructions attached as Exhibit A to hold in a non-interest bearing account until the Company consummates the IPO.

The consummation of the purchase and issuance of the Private Units shall occur simultaneously with the consummation of the IPO and the consummation of the purchase and issuance of the Over-Allotment Units shall occur simultaneously with the closing of any exercise of the over-allotment option related to the IPO. Simultaneously with the consummation of the IPO, Continental shall deposit the Private Unit Purchase Price, without interest or deduction, into the trust fund (“Trust Fund”) established by the Company for the benefit of the Company’s public shareholders as described in the Registration Statement. If the Company does not complete the IPO within ten (10) days from the date of this letter, the Private Unit Purchase Price (without interest or deduction) will be returned to the undersigned.

Each of the Company, and the undersigned acknowledges and agrees that Continental is serving hereunder solely as a convenience to the parties to facilitate the purchase of the Private Units and Continental’s sole obligation under this letter agreement is to act with respect to holding and disbursing the Private Unit Purchase Price as described above. Continental shall not be liable to the Company, Clear Street or the undersigned or any other person or entity in respect of any act or failure to act hereunder or otherwise in connection with performing its services hereunder unless Continental has acted in a manner constituting gross negligence or willful misconduct. The Company and the undersigned shall indemnify Continental against any claim made against it (including reasonable attorney’s fees) by reason of it acting or failing to act in connection with this letter agreement except as a result of its gross negligence or willful misconduct. Continental may rely and shall be protected in acting or refraining from acting upon any written notice, instruction or request furnished to it hereunder and believed by it to be genuine and to have been signed or presented by the proper party or parties.

The Private Units and Over-Allotment Units will be identical to the units to be sold by the Company in the IPO. Additionally, the undersigned agrees:

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to vote the Ordinary Shares included in the Private Units and Over-Allotment Units in favor of any proposed Business Combination;

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not to convert any Ordinary Shares included in the Private Units and Over-Allotment Units into the right to receive cash from the Trust Fund in connection with a shareholder vote to approve either a Business Combination or an amendment to the provisions of the Company’s Amended and Restated Memorandum and Articles of Association, and not to tender the Private Units and Over-Allotment Units in connection with a tender offer conducted prior to the closing of a Business Combination;

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the undersigned will not participate in any liquidation distribution with respect to the Private Units and Over-Allotment Units (but will participate in liquidation distributions with respect to any units or Ordinary Shares purchased by the undersigned in the IPO or in the open market) if the Company fails to consummate a Business Combination;

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that the Private Units, Over-Allotment Units and underlying securities will not be transferable until after the consummation of a Business Combination except (i) to the Company’s pre-IPO shareholders, or to the Company’s officers, directors, advisors and employees, (ii) transfers to the undersigned’s affiliates or its members upon its liquidation, (iii) to relatives and trusts for estate planning purposes, (iv) by virtue of the laws of descent and distribution upon death, (v) pursuant to a qualified domestic relations order, (vi) by private sales made in connection with the consummation of a Business Combination at prices no greater than the price at which the Private Units were originally purchased or (vii) to the Company for cancellation in connection with the consummation of a Business Combination, in each case (except for clause vii) where the transferee agrees to the terms of the transfer restrictions; and

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the Private Units and Over-Allotment Units will include any additional terms or restrictions as is customary in other similarly structured blank check company offerings or as may be reasonably required by the underwriters in the IPO in order to consummate the IPO, each of which will be set forth in the Registration Statement.

The undersigned acknowledges and agrees that the purchaser of the Private Units and Over-Allotment Units will execute agreements in form and substance typical for transactions of this nature necessary to effectuate the foregoing agreements and obligations prior to the consummation of the IPO as are reasonably acceptable to the undersigned, including but not limited to an insider letter.

The undersigned hereby represents and warrants that:

(a)

it has been advised that the Private Units and Over-Allotment Units have not been registered under the Securities Act;

(b)

it will be acquiring the Private Units and Over-Allotment Units for its account for investment purposes only and not with a view to the distribution or resale of such units;

(c)

it has no present intention of selling or otherwise disposing of the Private Units and Over-Allotment Units in violation of the securities laws of the United States;

(d)

it is an “accredited investor” as defined by Rule 501 of Regulation D promulgated under the Securities Act;

(e)

it has had both the opportunity to ask questions and receive answers from the officers and directors of the Company and all persons acting on its behalf concerning the terms and conditions of the offer made hereunder;

(f)

it is familiar with the proposed business, management, financial condition and affairs of the Company;

(g)

it has full power, authority and legal capacity to execute and deliver this letter and any documents contemplated herein or needed to consummate the transactions contemplated in this letter;

(h)

it has he financial ability to bear the economic risk of its investment in the Private Units and the Over-Allotment Units and is able to bear a total loss of its investment in such units;

(i)

it understands that the Private Units and Over-Allotment units are not readily marketable;

(j)

it has no need for liquidity with respect to its investment in the Private units and the Over-Allotment Units and no need to dispose of any portion thereof to satisfy any existing or contemplated undertaking or indebtedness;

(k)

it is capable of assessing the merits of and understanding (on its own behalf or through independent professional advice), and understands and accepts, the terms, conditions and risks of its investment in the Private Units and the Over-Allotment Units; and

(l)

this letter constitutes its legal, valid and binding obligation, and is enforceable against it.

This letter agreement constitutes the entire agreement between the undersigned and the Company with respect to the purchase of the Private Units and Over-Allotment Units, and supersedes all prior and contemporaneous understandings, agreements, representations and warranties, both written and oral, with respect to the same.

Very truly yours,

AA Mission Sponsor II

By:	/s/ ▇▇▇▇ ▇▇▇
Name:	▇▇▇▇ ▇▇▇
Title:	Managing Member

Accepted and Agreed:

AA Mission Acquisition Corp. II

By:	/s/ ▇▇▇▇ ▇▇▇
Name:	▇▇▇▇ ▇▇▇
Title:	Chief Executive Officer and Director

Private Placement Unit Purchase Agreement

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