The term of the member composing Class I shall correspond to the Mayor's official tenure or if the member is the Mayor's designee in the absence of the Mayor, the designee shall serve at the pleasure of the Mayor during the Mayor's official tenure. The terms of the members composing Class II and Class III shall be for one year or terminate at the completion of their respective terms of office, whichever occurs first, except for a Class II member who is also a member of the Environmental Commission. The term of a Class II or Class IV member who is also a member of the Environmental Commission shall be for three years or terminate at the completion of their term of office as a member of the Environmental Commission, whichever comes first. The term of a Class IV member who is also a member of the Board of Adjustment or Board of Education shall terminate whenever he/she is no longer a member of such other body or at the completion of his/her Class IV term, whichever occurs first. Thereafter, the Class IV term of each such Class IV member shall be four years. If a vacancy in any class shall occur otherwise than by expiration of the Planning Board term, it shall be filled by appointment, as above provided, for the unexpired term.