The term of the member composing Class I shall correspond to the Class I member's official tenure. The terms of the members composing Class II shall be for one year or terminate at the completion of their respective terms of office, whichever occurs first. The term of the Class III member shall be for one year or the termination of their office, whichever occurs first. The terms of all Class IV members first appointed under this chapter shall be so determined that to the greatest practicable extent the expiration of such terms shall be distributed evenly over the first four years after their appointment, provided that the initial Class IV term of no member shall exceed four years. Thereafter, the Class IV term of each such member shall be four years. If a vacancy in any class shall occur otherwise than by expiration of the Joint Land Use Board term, it shall be filled by appointment, as above provided, for the unexpired term.