SECTION 4. Adjustments to allowable state tax revenues

Allowable state tax revenues for a fiscal year shall be reduced, if, after the effective date of this chapter, by an enactment of the general court, authority is granted to local governmental units by local option or otherwise to impose or levy a new, or to increase an existing, tax or excise. The amount by which allowable state tax revenues for such fiscal year shall be reduced shall be as nearly as possible equal to the additional amount of revenues to be derived by local governmental units by the tax or excise as determined by the Commissioner. Provided, however, that any reduction in allowable state tax revenues for a fiscal year required by this subsection shall first occur in the allowable state tax revenues for the fiscal year beginning immediately after the effective date of the enactment of the general court which gives rise to the reduction herein required.