Exhibit 4.1

. ZQ|CERT#|COY|CLS|RGSTRY|ACCT#|TRANSTYPE|RUN#|TRANS# COMMON STOCK COMMON STOCK PO PAR VALUE $0.001 MR ADD ADD ADD ADD 43 2 1 A Box DESIGNATION SAMPLE 43004, Certificate Shares (IF Number * * 000000 * * * 000000 * ANY) ZQ00000000 000000 Providence * 000000 * RI Upstream Bio, Inc. 000000 INCORPORATED UNDER THE LAWS OF THE STATE OF DELAWARE 02940 Mr. Alexander David Sample Mr. Alexander David Sample Mr. Alexander David Sample Mr. Alexander David Sample Mr. Alexander David Sample SEE REVERSE FOR CERTAIN DEFINITIONS—Mr. Alexander David Sample Mr. Alexander David Sample Mr. Alexander David Sample Mr. Alexander David Sample Mr. Alexander David THIS CERTIFIES THAT Sample Mr. Alexander David Sample Mr. Alexander David Sample Mr. Alexander David Sample Mr. Alexander David Sample Mr. Alexander David Sample Mr MR . Alexander.David SAMPLE Sample Mr. Alexander David &Sample MRS Mr. Alexander . SAMPLE David Sample Mr. Alexander & David Sample Mr. 3004 Alexander David Sample Mr. Alexander David Sample Mr. Alexander David Sample Mr Alexander David Sample Mr. Alexander David Sample CUSIP XXXXXX XX X Mr. Alexander David Sample Mr. Alexander David Sample Mr. Alexander David Sample Mr. Alexander David Sample Mr. Alexander David Sample Mr. Alexander David Sample Mr. Alexander David Sample Mr. Alexander David Sample Mr. Alexander David Sample Mr. Alexander David Sample Mr. Alexander MR David Sample . SAMPLE Mr. Alexander David Sample &Mr . Alexander MRS David Sample . SAMPLE Mr. Alexander David Sample Mr. Alexander David Sample Mr. Alexander David Sample Mr. Alexander David Sample Mr. Alexander David Sample Mr. Alexander David Sample Mr. Alexander David Sample Mr. Alexander David Sample Mr. Alexander David Sample Mr. Alexander David Sample Mr. Alexander David Sample Mr. Alexander David Sample Mr. Alexander David Sample Mr. Alexander David Sample Mr. Alexander David Sample Mr. Sample Mr. Sample is the owner of 000000 Shares 000000 Shares 000000 Shares 000000 Shares 000000 Shares 000000 Shares 000000 Shares 000000 Shares * *000000 Shares 000000 Shares 000000 Shares 000000 Shares 000000 Shares 000000 Shares 000000 Shares 000000 Shares 000000 Shares 000000 Shares 000000 Shares 000000 Shares 000000 Shares 000000 Shares 000000 Shares 000000 Shares 0 THIS CERTIFICATE IS TRANSFERABLE IN 00000 Shares 000000 Shares 000000 Shares 000000 Shares 000000 Shares 000000 Shares 000000 Shares 000000 Shares 00 *ZERO HUNDRED THOUSAND 0000 Shares 000000 Shares 000000 Shares 000000 Shares 000000 Shares 000000 Shares 000000 Shares 000000 Shares 000 CITIES DESIGNATED BY THE TRANSFER 000 Shares 000000 Shares 000000 Shares 000000 Shares 000000 Shares 000000 Shares 000000 Shares 000000 Shares 0000 AGENT, AVAILABLE ONLINE AT 00 Shares 000000 Shares 000000 Shares 000000 Shares 000000 Shares 000000 Shares 000000 Shares 000000 Shares 00000 0 Shares 000000 Shares 000000 Shares 000000 Shares 000000 Shares 000000 Shares 000000 Shares 000000 Shares 000000 ZERO HUNDRED AND ZERO * www.computershare.com Shares 000000 Shares 000000 Shares 000000 Shares 000000 Shares 000000 Shares 000000 Shares 000000 Shares 000000* *Shares 000000 Shares 000000 Shares 000000 Shares 000000 Shares 000000 Shares 000000 Shares 000000 Shares 000000 Shares 000000 Shares 000000 Shares 000000 Shares 000000 Shares 000000 Shares 000000 Shares 000000 Shares 000000 S FULLY-PAID AND NON-ASSESSABLE SHARES OF COMMON STOCK OF Upstream Bio, Inc. (hereinafter called the “Company”), transferable on the books of the Company in person or Total DTC by duly authorized attorney, upon surrender of this Certificate properly endorsed. This Certificate and the shares Holder represented hereby, are issued and shall be held subject to all of the provisions of the Certificate of Incorporation, Number Certificateof Insurance ID as amended, and the By-Laws, as amended, of the Company (copies of which are on file with the Company and Value with the Transfer Agent), to all of which each holder, by acceptance hereof, assents. This Certificate is not valid Transaction Shares CUSIP/IDENTIFIER unless countersigned and registered by the Transfer Agent and Registrar. Numbers 1234567890/1234567890 1234567890/1234567890 1234567890/1234567890 1234567890/1234567890 1234567890/1234567890 1234567890/1234567890 Witness the facsimile seal of the Company and the facsimile signatures of its duly authorized officers. DATED DD-MMM-YYYY AM B E I COUNTERSIGNED AND REGISTERED: 6 5 4 3 2 1 FACSIMILE SIGNATURE TO COME R O, 12345678 T O RP RA I S O T N COMPUTERSHARE TRUST COMPANY, N.A. Num/No P C E . President U C. TRANSFER AGENT AND REGISTRAR, 6 5 4 3 2 1 Denom April 21, 2021 . XXXXXX DEL RE 1,000,000 FACSIMILE SIGNATURE TO COME AWA 7 6 5 4 3 2 1 . XX Total 123456789012345 123456 00 XXXXXXXXXX X By Secretary AUTHORIZED SIGNATURE

UPSTREAM BIO, INC. THE COMPANY WILL FURNISH WITHOUT CHARGE TO EACH SHAREHOLDER WHO SO REQUESTS, A SUMMARY OF THE POWERS, DESIGNATIONS, PREFERENCES AND RELATIVE, PARTICIPATING, OPTIONAL OR OTHER SPECIAL RIGHTS OF EACH CLASS OF STOCK OF THE COMPANY AND THE QUALIFICATIONS, LIMITATIONS OR RESTRICTIONS OF SUCH PREFERENCES AND RIGHTS, AND THE VARIATIONS IN RIGHTS, PREFERENCES AND LIMITATIONS DETERMINED FOR EACH SERIES, WHICH ARE FIXED BY THE CERTIFICATE OF INCORPORATION OF THE COMPANY, AS AMENDED, AND THE RESOLUTIONS OF THE BOARD OF DIRECTORS OF THE COMPANY, AND THE AUTHORITY OF THE BOARD OF DIRECTORS TO DETERMINE VARIATIONS FOR FUTURE SERIES. SUCH REQUEST MAY BE MADE TO THE OFFICE OF THE SECRETARY OF THE COMPANY OR TO THE TRANSFER AGENT. THE BOARD OF DIRECTORS MAY REQUIRE THE OWNER OF A LOST OR DESTROYED STOCK CERTIFICATE, OR HIS LEGAL REPRESENTATIVES, TO GIVE THE COMPANY A BOND TO INDEMNIFY IT AND ITS TRANSFER AGENTS AND REGISTRARS AGAINST ANY CLAIM THAT MAY BE MADE AGAINST THEM ON ACCOUNT OF THE ALLEGED LOSS OR DESTRUCTION OF ANY SUCH CERTIFICATE. The following abbreviations, when used in the inscription on the face of this certificate, shall be construed as though they were written out in full according to applicable laws or regulations: TEN COM—as tenants in common UNIF GIFT MIN ACT -............................................Custodian (Cust) (Minor) TEN ENT—as tenants by the entireties under Uniform Gifts to Minors Act (State) JT TEN—as joint tenants with right of survivorship UNIF TRF MIN ACT -............................................Custodian (until age................................ ) and not as tenants in common (Cust) .............................under Uniform Transfers to Minors Act (Minor) (State) Additional abbreviations may also be used though not in the above list. PLEASE INSERT SOCIAL SECURITY OR OTHER IDENTIFYING NUMBER OF ASSIGNEE For value received,____________________________ hereby sell, assign and transfer unto ________________________________________________________________________________________________________________________________ (PLEASE PRINT OR TYPEWRITE NAME AND ADDRESS, INCLUDING POSTAL ZIP CODE, OF ASSIGNEE) ________________________________________________________________________________________________________________________________ ________________________________________________________________________________________________________________________________ _______________________________________________________________________________________________________________________ Shares of the common stock represented by the within Certificate, and do hereby irrevocably constitute and appoint _______________________________________________________________________________________________________________________ Attorney to transfer the said stock on the books of the within-named Company with full power of substitution in the premises. Dated: __________________________________________ 20__________________ Signature(s) Guaranteed: Medallion Guarantee Stamp THE SIGNATURE(S) SHOULD BE GUARANTEED BY AN ELIGIBLE GUARANTOR INSTITUTION (Banks, Stockbrokers, Savings and Loan Associations and Credit Unions) WITH MEMBERSHIP IN AN APPROVED SIGNATURE GUARANTEE MEDALLION PROGRAM, PURSUANT TO S.E.C. RULE 17Ad-15. Signature:____________________________________________________________ Signature:____________________________________________________________ Notice: The signature to this assignment must correspond with the name as written upon the face of the certificate, in every particular, without alteration or enlargement, or any change whatever.