Class IV: Six (6) citizens of the Borough of Somerville to be appointed by the Mayor. The members of Class IV shall hold no other municipal office, position or employment, except that one such member may be a member of the Zoning Board of Adjustment or Historic Preservation Commission, if one exists, and one such member may be a member of the Board of Education. The member of the Environmental Commission who is also a member of the Planning Board as required by P.L. 1968, c. 245 shall be a Class IV Planning Board member, unless there be among the Class IV or alternate members of the Board both a member of the Zoning Board of Adjustment or Historic Preservation Commission, if one exists, and a member of the Board of Education, in which case the member common to the Planning Board and Environmental Commission shall be deemed a Class II member of the Board. For the purpose of this qualification, membership on a municipal board or commission whose function is advisory in nature, and the establishment of which is discretionary and not required by statute, shall not be considered the holding of municipal office.