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SCHEDULE 13D/A 0001055624 XXXXXXXX LIVE 4 Common Stock, $0.0001 par value per share 04/03/2025 false 0001681087 878972108 Tectonic Therapeutic, Inc. 490 Arsenal Way Suite 210 Watertown MA 02472 Michael K. Bradshaw, Jr. (202) 689-2800 Nelson Mullins Riley & Scarborough LLP 101 Constitution Avenue NW, Suite 900 Washington DC 20001 0001055624 N Timothy A. Springer WC PF N X1 4313558.00 0.00 4313558.00 0.00 4313558.00 N 31.5 IN OO The percentages of beneficial ownership reported on this page are based on a total of 18,662,672 Shares issued and outstanding as of March 1, 2025, as reported by the Issuer in its Annual Report on Form 10-K for the year ended December 31, 2024, filed with the SEC on March 20, 2025. 0001677543 N TAS Partners LLC WC N DE 1382723.00 0.00 1382723.00 0.00 1382723.00 N 7.4 OO The percentages of beneficial ownership reported on this page are based on a total of 18,662,672 Shares issued and outstanding as of March 1, 2025, as reported by the Issuer in its Annual Report on Form 10-K for the year ended December 31, 2024, filed with the SEC on March 20, 2025. Y Chafen Lu WC PF N X1 186134.00 0.00 186134.00 0.00 186134.00 N 1.0 IN OO The percentages of beneficial ownership reported on this page are based on a total of 18,662,672 Shares issued and outstanding as of March 1, 2025, as reported by the Issuer in its Annual Report on Form 10-K for the year ended December 31, 2024, filed with the SEC on March 20, 2025. Common Stock, $0.0001 par value per share Tectonic Therapeutic, Inc. 490 Arsenal Way Suite 210 Watertown MA 02472 This Amendment No. 4 to the statement on Schedule 13D (this "Statement") relates to the shares of common stock, par value $0.0001 per share (the "Shares"), of Tectonic Therapeutic, Inc., a Delaware corporation (the "Issuer"), and amends the initial statement on Schedule 13D originally filed by Timothy A. Springer ("Dr. Springer") and TAS Partners LLC ("TAS") on June 26, 2024, as amended by Amendment No. 1 filed on September 26, 2024 adding Chafen Lu ("Dr. Lu" and, collectively with Dr. Springer and TAS, the "Reporting Persons") as a reporting person, Amendment No. 2 filed on October 24, 2024 and Amendment No. 3 filed on February 10, 2025 (such statement and amendments, as further amended herein, the "Schedule 13D"). Each Item below amends and supplements the information disclosed under the corresponding Item of the Schedule 13D. Except as specifically provided herein, this Amendment No. 4 does not modify any of the information previously reported in the Schedule 13D. Capitalized terms used but not defined in this Amendment No. 4 shall have the same meanings herein as are ascribed to such terms in the Schedule 13D. N/A N/A N/A N/A N/A N/A Item 3 of the Schedule 13D is hereby amended to be supplemented by the following: On April 3, 2025, Dr. Springer and TAS entered into Stock Purchase Agreements with Polaris Partners IX, L.P. ("Polaris"), pursuant to which Dr. Springer purchased 87,500 Shares from Polaris at a purchase price of $17.10 per share, or $1,496,250 in the aggregate, payable in cash, and TAS purchased 412,500 Shares from Polaris at a purchase price of $17.10 per share, or $7,053,750 in the aggregate, payable in cash. Each of Dr. Springer and TAS represented that he or it was an "accredited investor," as defined in Regulation D, and was acquiring such Shares for investment only and not with a view towards, or for resale in connection with, the public sale or distribution thereof. The closing occurred on April 7, 2025. N/A Item 5 of Schedule 13D is hereby amended and restated in full as follows: The information set forth under Item 3 and the cover page of the Schedule 13D is incorporated herein by reference into this Item 5. The percentages of beneficial ownership reported in this Item 5, and on each Reporting Person's cover page to this Statement, are based on a total of 18,662,672 Shares issued and outstanding as of March 1, 2025, as reported by the Issuer in its Annual Report on Form 10-K for the year ended December 31, 2024, filed with the SEC on March 20, 2025. All of the Share numbers reported below, and on each Reporting Person's cover page to this Statement, are as of April 3, 2025. The Reporting Persons, in the aggregate, beneficially own 5,882,415 Shares, representing approximately 31.5% of such class of securities. Dr. Springer is the beneficial owner of a total of 5,882,415 Shares, representing approximately 31.5% of the outstanding Shares and consisting of (i) 4,313,558 Shares held directly, (ii) 1,382,723 Shares held by TAS and (iii) 186,134 Shares held by Dr. Lu. TAS is the beneficial owner of a total of 1,382,773 Shares, representing approximately 7.4% of the outstanding Shares. TAS holds all such Shares directly. Dr. Springer is the sole managing member of TAS. Dr. Lu is the beneficial owner of a total of 186,134 Shares, representing approximately 1.0% of the outstanding Shares. Dr. Lu holds all such Shares directly. Dr. Lu is the spouse of Dr. Springer. Dr. Springer exercises sole voting and dispositive power over the Shares held by him directly and the Shares held by TAS. Dr. Springer disclaims beneficial ownership of the Shares held by TAS, except to the extent of his pecuniary interest therein. Dr. Lu exercises sole voting and dispositive power over the Shares held by her directly. The Reporting Persons have not engaged in any transactions with respect to the Shares during the 60 days before the date of this filing, except as described herein and in Item 3 of Schedule 13D. No person other than the Reporting Persons is known to have the right to receive or the power to direct the receipt of dividends from, or the proceeds from the sale of, the Shares. Not applicable. N/A Timothy A. Springer /s/ Timothy A. Springer Timothy A. Springer 04/07/2025 TAS Partners LLC /s/ Timothy A. Springer Timothy A. Springer/Manager 04/07/2025 Chafen Lu /s/ Chafen Lu Chafen Lu 04/07/2025