The term of the member composing Class I shall correspond to his official tenure. If the member is the Mayor's designee, the designee shall serve at the pleasure of the Mayor during the Mayor's 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 his term of office as a member of the Environmental Commission, whichever occurs 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 Class IV term, 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 member shall be four years. Nothing contained herein shall affect the term of any present member of the Planning Board, all of whom shall continue on office until the completion of the terms for which they were appointed. All terms shall run from January 1 of the year in which the appointment is made.