4. Each of the counties of the region that has not become a participating county at the time of filing of the certificate required by subdivision three of this section may do so by enacting enabling legislation, a duly certified copy of which must be filed with the board of directors, the commission, the secretary of state and the county clerk of each participating county. In the event that a county elects to participate after June first, nineteen hundred ninety, the effective date of approval by the commission shall not be earlier than the date that branch offices are established and operating. If, at the time of such election, the commission has approved a plan of operation for the corporation, a county may not become a participating county without approval by the commission of a modified feasibility study and amended plan of operation which shall be submitted by the corporation to the commission pursuant to section five hundred twenty-one of this chapter. If the participating counties in the region have contributed or loaned funds or other consideration to the corporation, the board of directors may require that any county subsequently electing to become a participating county make such contributions in the same proportion, if any, as may have governed such contributions or loans by participating counties. Any dispute as to the value of consideration or as to a contribution required by the board of directors shall be resolved by the commission.