§ 431. Salary base for computing retirement benefits. In any retirement or pension plan to which the state or municipality thereof contributes, the salary base for the computation of retirement benefits shall in no event include any of the following earned or received, on or after April first, nineteen hundred seventy-two:

1. lump sum payments for deferred compensation, sick leave, accumulated vacation or other credits for time not worked,

2. any form of termination pay,

3. any additional compensation paid in anticipation of retirement, or

4. that portion of compensation earned during any twelve months included in such salary base period which exceeds that of the preceding twelve months by more than twenty per centum.