The terms of the members first appointed under this subsection shall be so determined that, to the greatest practicable extent, the expiration of the terms shall be distributed, in the case of regular members, evenly over the first four years after their appointment and, in the case of alternate members, evenly over the first two years after their appointment, provided that the initial term of no regular member shall exceed four years and that initial term of no alternate member shall exceed two years. Thereafter, the term of a regular member shall be four years, and the term of an alternate member shall be two years. A vacancy occurring other than by expiration of term shall be filled for the unexpired term only. A term of a Planning Board member or a Zoning Board member shall coincide with the term of such member on the Planning Board or Zoning Board. Vacancies shall be filled in the same manner as the original appointment and shall be only for the balance of the unexpired term.