Municipal police member coverage plan 4 is applicable to municipal police members of an affiliated public employer on the first day of the calendar month following certification of the election adopting municipal police member coverage plan 4 by an affirmative vote of the majority of the affiliated public employer’s municipal police members. The election shall be conducted by the affiliated public employer. The certification shall be in the form prescribed by the retirement board. The election procedures shall afford all municipal police members of the affiliated public employer an opportunity to vote. An election adopting municipal police member coverage plan 4 for a given affiliated public employer is irrevocable for the purpose of subsequently adopting a coverage plan which would decrease employer or employee contributions with respect to all current and future municipal police members of that affiliated public employer.