Four (4) Class IV alternate members shall be appointed to the Planning Board by the Mayor, and shall meet the qualifications of Class IV members of the Planning Board. Alternate members shall be designated at the time of appointment by the Mayor as Alternate No. 1, Alternate No. 2, Alternate No. 3, and Alternate No. 4. The terms of the alternate members shall be for two years, except that the terms of the alternate members shall be such that the term of not more than two (2) alternate member shall expire in any one year; provided, however, that in no instance shall the initial term of the alternate members first appointed exceed three years. A vacancy occurring otherwise than by expiration of a term shall be filled by the Mayor for the unexpired term only. Alternate members may participate in discussions of the proceedings but may not vote except in the absence or disqualification of a regular member of any class. A vote shall not be delayed in order that a regular member may vote instead of an alternate member. In the event that a choice must be made as to which alternate member is to vote, Alternate No. 1 shall vote, if present and if not, Alternate No. 2 shall vote; in the event Alternate No. 1 and 2 are not present, Alternate No 3 shall vote, and in the event Alternates 1, 2 and 3 are not present Alternate No. 4 shall vote.