SCHEDULE 13G/A 0001687968 XXXXXXXX LIVE 1 Common Stock, par value $0.001 per share 12/31/2025 0001501697 X4 PHARMACEUTICALS, INC. 98420X202 61 North Beacon Street 4th Floor Boston MA 02134 Rule 13d-1(c) Bain Capital Life Sciences Fund, L.P. E9 0 0 0 0 0 N 0 PN Bain Capital Life Sciences Fund II, L.P. E9 0 64190 0 64190 64190 N 0.07 PN BCIP Life Sciences Associates, LP DE 0 7817 0 7817 7817 N 0.01 PN BCLS II Investco, LP DE 0 758598 0 758598 758598 N 0.8 PN BCLS I Investco, LP DE 0 89633 0 89633 89633 N 0.09 PN BCLS II Equity Opportunities, LP DE 0 7047216 0 7047216 7047216 N 7.43 PN X4 PHARMACEUTICALS, INC. 61 North Beacon Street, 4th Floor, Boston, Massachusetts 02134. This Schedule 13G is being filed jointly by Bain Capital Life Sciences Fund, L.P., a Cayman Islands exempted limited partnership ("BCLS Fund I"), Bain Capital Life Sciences Fund II, L.P., a Cayman Islands exempted limited partnership ("BCLS Fund II"), BCIP Life Sciences Associates, LP, a Delaware limited partnership ("BCIPLS"), BCLS II Investco, LP, a Delaware limited partnership ("BCLS II Investco"), BCLS I Investco, LP, a Delaware limited partnership ("BCLS I Investco"), and BCLS II Equity Opportunities, LP, a Delaware limited partnership ("BCLS II Equity" and, together with BCLS Fund I, BCLS Fund II, BCIPLS, BCLS II Investco and BCLS I Investco, the "Reporting Persons"). Bain Capital Life Sciences Investors, LLC, a Delaware limited liability company ("BCLSI"), is (i) the general partner of Bain Capital Life Sciences Partners, LP, a Cayman Islands exempted limited partnership ("BCLSP"), which is the general partner of BCLS Fund I, and (ii) the manager of Bain Capital Life Sciences Investors II, LLC, a Cayman Islands limited liability company ("BCLSI II"), which is the general partner of BCLS Fund II. Boylston Coinvestors, LLC, a Delaware limited liability company ("Boylston"), is the general partner of BCIPLS. BCLSI governs the investment strategy and decision-making process with respect to investments held by BCIPLS. BCLS II Investco (GP), LLC, a Delaware limited liability company ("BCLS II Investco GP"), whose manager is BCLS Fund II, is the general partner of BCLS II Investco. BCLS I Investco GP, LLC, a Delaware limited liability company ("BCLS I Investco GP"), whose manager is BCLS Fund I, is the general partner of BCLS I Investco. BCLS II Equity Opportunities GP, LLC, a Delaware limited liability company ("BCLS II Equity GP" and, together with the Reporting Persons, BCLSI, BCLSP, BCLSI II, Boylston, BCLS II Investco GP and BCLS I Investco GP, the "Bain Capital Life Sciences Entities"), whose manager is BCLS Fund II, is the general partner of BCLS II Equity. As a result, BCLSI may be deemed to share voting and dispositive power with respect to the securities held by the Reporting Persons. The Reporting Persons have entered into a Joint Filing Agreement, dated February 17, 2026, pursuant to which the Reporting Persons have agreed to file this Schedule 13G jointly in accordance with the provisions of Rule 13d-1(k)(1) promulgated under the Act. The principal business address for each of the Bain Capital Life Sciences Entities is 200 Clarendon Street, Boston, Massachusetts 02116. See Item 2(a) hereof. Y As of December 31, 2025, (i) BCLS Fund I directly holds zero shares of Common Stock; (ii) BCLS Fund II directly holds 64,190 shares of Common Stock; (iii) BCIPLS directly holds 7,817 shares of Common Stock; (iv) BCLS II Investco directly holds 543,642 shares of Common Stock, a warrant exercsiable for up to 214,956 shares of Common Stock and a Class C warrant exercisable for up to 68,818 shares of Common Stock; (v) BCLS I Investco directly holds a warrant exercisable for up to 89,633 shares of Common Stock and a Class C warrant exercisable for up to 28,696 shares of Common Stock; and (vi) BCLS II Equity directly holds a pre-funded warrant exercisable for up to 7,047,216 shares of Common Stock. BCLS II Investco, BCLS I Investco and BCLS II Equity are (i) prohibited from exercising warrants to purchase Common Stock or pre-funded warrants to purchase Common Stock if, as a result of such exercise, the Reporting Persons would together beneficially own more than 9.99% of the total number of shares of Common Stock then issued and outstanding immediately after giving effect to the exercise and (ii) prohibited from exercising Class C warrants to purchase Common Stock if, as a result of such exercise, the Reporting Persons would together beneficially own more than 4.99% of the total number of shares of Common Stock then issued and outstanding immediately after giving effect to the exercise (collectively, the "Beneficial Ownership Blockers"). Accordingly, pursuant to Rule 13d-3 of the Act and the relationships described herein, as of December 31, 2025, the Reporting Persons may be deemed to collectively beneficially own (i) 615,649 shares of Common Stock, (ii) a pre-funded warrant exercisable for up to 7,047,216 shares of Common Stock and (iii) warrants exercisable for up to 304,589 shares of Common Stock, together representing 8.41% of the oustanding shares of Common Stock. The calculation of the beneficial ownership of the Reporting Persons is based on (i) 87,436,688 shares of Common Stock issued and outstanding as of October 31, 2025, as reported by the Issuer in its Quarterly Report on Form 10-Q filed with the Securities and Exchange Commission on November 5, 2025, and (ii) 7,351,805 shares of Common Stock issuable upon the exercise of warrants and pre-funded warrants held by the Reporting Persons, which reflects the Beneficial Ownership Blockers. See Item 4(a) hereof. 0 See Item 4(a) hereof. 0 See Item 4(a) hereof. Y Y Y Y Y N By signing below I certify that, to the best of my knowledge and belief, the securities referred to above were not acquired and are not held for the purpose of or with the effect of changing or influencing the control of the issuer of the securities and were not acquired and are not held in connection with or as a participant in any transaction having that purpose or effect, other than activities solely in connection with a nomination under ?? 240.14a-11. Bain Capital Life Sciences Fund, L.P. /s/ Andrew Hack 02/17/2026 Bain Capital Life Sciences Fund II, L.P. /s/ Andrew Hack 02/17/2026 BCIP Life Sciences Associates, LP /s/ Andrew Hack 02/17/2026 BCLS II Investco, LP /s/ Andrew Hack 02/17/2026 BCLS I Investco, LP /s/ Andrew Hack 02/17/2026 BCLS II Equity Opportunities, LP /s/ Andrew Hack 02/17/2026