The term of the member of Class I shall correspond to his official tenure. The terms of the members of Class II and Class III shall be for one year, or terminate at the completion of their respective terms of office, whichever comes first. The terms of the members of Class IV, first appointed, shall be one, two, three, and four years. Thereafter, the term of each such member shall be four years. The terms of alternate members of Class IV, first appointed, shall be one and two years, respectively, and thereafter for two years. If a vacancy in any class shall occur otherwise than by expiration of the term, it shall be filled by appointment, as above provided, for the unexpired term.