Sec Form 4 Filing - Ciongoli Gregory Austin @ Atara Biotherapeutics, Inc. - 2026-06-09

FORM 4 Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940

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1. Name and Address of Reporting Person Ciongoli Gregory Austin 2. Issuer Name and Ticker or Trading Symbol Atara Biotherapeutics, Inc. [ ATRA] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) X __ Director X __ 10% Owner _____ Officer (give title below) X __ Other (specify below) See Remarks (Last) (First) (Middle) C/O ATARA BIOTHERAPEUTICS, INC., 1280 RANCHO CONEJO BOULEVARD 3. Date of Earliest Transaction (MM/DD/YY) 06/09/2026 (Street) THOUSAND OAKS, CA91320 4. If Amendment, Date Original Filed (MM/DD/YY) 6. Individual or Joint/Group Filing (Check Applicable Line) __ X __ Form filed by One Reporting Person _____ Form filed by More than One Reporting Person (City) (State) (Zip)

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned 1.Title of Security (Instr. 3) 2. Transaction Date (MM/DD/YY) 2A. Deemed Execution Date, if any (MM/DD/YY) 3. Transaction Code (Instr. 8) 4. Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5) 5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4) 6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4) 7. Nature of Indirect Beneficial Ownership (Instr. 4) Code V Amount (A) or (D) Price Common Stock 06/09/2026 A 12,000 ( 1 ) A $ 0 12,000 D Common Stock 1,209,395 I See Footnote ( 2 )

Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned ( e.g. , puts, calls, warrants, options, convertible securities) 1. Title of Derivative Security (Instr. 3) 2. Conversion or Exercise Price of Derivative Security 3. Transaction Date (MM/DD/YY) 3A. Deemed Execution Date, if any (MM/DD/YY) 4. Transaction Code (Instr. 8) 5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4, and 5) 6. Date Exercisable and Expiration Date (MM/DD/YY) 7. Title and Amount of Underlying Securities (Instr. 3 and 4) 8. Price of Derivative Security (Instr. 5) 9. Number of Derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4) 10. Ownership Form of Derivative Security: Direct (D) or Indirect (I) (Instr. 4) 11. Nature of Indirect Beneficial Ownership (Instr. 4) Code V (A) (D) Date Exercisable Expiration Date Title Amount or Number of Shares

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other Ciongoli Gregory Austin C/O ATARA BIOTHERAPEUTICS, INC. 1280 RANCHO CONEJO BOULEVARD THOUSAND OAKS, CA91320 X X See Remarks Signatures /s/ John Chao, Attorney-in-Fact for Gregory A. Ciongoli 06/11/2026 Signature of Reporting Person Date

Explanation of Responses:

( 1 )These restricted stock units shall vest on the earlier of June 9, 2027 or the date of the next annual meeting of stockholders, subject to the Reporting Person's continuous service. ( 2 )The securities reported on this row are directly held by Adiumentum Capital Fund I LP, a limited partnership organized under the laws of Delaware ("Adiumentum"). Adiumentum Capital Fund I GP LLC, a limited liability company organized under the laws of Delaware ("Adiumentum GP"), is the general partner of Adiumentum. Gregory A. Ciongoli is the managing partner of Adiumentum and the managing member of Adiumentum GP. Each of Adiumentum, Adiumentum GP, and Mr. Ciongoli may be deemed to have the shared power to vote or direct the vote (and the shared power to dispose or direct the disposition of) the securities that are directly held by Adiumentum. Each of Adiumentum, Adiumentum GP, and Mr. Ciongoli may be deemed to have a pecuniary interest in the securities that are directly held by Adiumentum, but disclaims any beneficial ownership of such securities, except to the extent of any pecuniary interest therein.

Remarks: Exhibit List: Exhibit 24 - Power of Attorney. Mr. Ciongoli serves as a director on the Board of Directors of the Issuer and, as a result, Adiumentum and Adiumentum GP are directors by deputization for purposes of Section 16 of the Securities Exchange Act of 1934, as amended. Neither Adiumentum nor Adiumentum GP have a pecuniary interest in the restricted stock units awarded to Mr. Ciongoli as reported in this Form 4, and therefore Adiumentum and Adiumentum GP have been excluded as reporting persons from this Form 4.

Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.

* If the form is filed by more than one reporting person, see Instruction 4(b)(v).

** Intentional misstatements or omissions of facts constitute Federal Criminal Violations. See 18 U.S.C. 1001 and 15 U.S.C. 78ff(a).

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