Notwithstanding any other provisions of this chapter to the contrary, a member may retire after completing four years of creditable service if he becomes totally and permanently disabled after commencing service as a probate judge or as an employee of the board. Any such probate judge or employee shall be entitled to receive retirement benefits in the amount that he would receive if his retirement were effective at the time he became disabled. All questions relating to the degree and nature of the total and permanent disability suffered by the probate judge or employee shall be determined by the board.