4. Any project labor agreement negotiated pursuant to this act between the public entity or its representative or a construction manager and one or more labor organizations shall be binding on all contractors and subcontractors working on the public works project and may include provisions that permit contractors and subcontractors working on the public works project to retain a percentage of their current workforce, and provisions that the successful bidder and any subcontractor of the bidder need not be a party to a labor agreement with the labor organizations other than for the public works project covered by the project labor agreement. Each project labor agreement shall stipulate that:
a. the provisions of the project labor agreement shall apply to work done at construction sites of the public works project and shall not apply to work done outside of those sites; and
b. if a union trust fund covered by the terms and conditions of the project labor agreement has not adopted the building and construction industry exemption authorized by subsection (b) of section 4203 of the Employee Retirement Income Security Act of 1974 (29 U.S.C. s.1383(b)), the signatory employers shall not be obligated to hire employees covered by that fund.