There shall be four alternate members appointed to the Land Use Board as Class IV members. The Mayor shall appoint said alternate members, and they shall meet the qualification of Class IV members as hereinafter provided. Said alternate members shall be designated at the time of their appointment by the Mayor as "Alternate No. 1," "Alternate No. 2," "Alternate No. 3," and "Alternate No. 4." Alternate members of Class IV shall serve a for a term of two years, except that the terms of the alternate members shall be such that the term of not more than one alternate member shall expire in any one year; provided, however, that the term of not more than two alternate members shall expire in any one year; and provided further that in no instance shall the terms of the alternate members first appointed exceed two years.