The members of the probation commission shall hold office for four years and until their successors are appointed and qualify. Of those first appointed, however, one shall hold office for one year, two for two years, two for three years, and two for four years; and the respective terms of the members first appointed shall be determined by lot as soon as possible after their appointment. When a vacancy occurs in a probation commission by expiration of the term of office of any member thereof, his or her successor shall be appointed to hold office for the term of four years. When a vacancy occurs for any other reason the appointee shall hold office for the unexpired term of his or her predecessor.