4. Any vacancy, other than a casual vacancy, occurring in the office of a member appointed under the foregoing provisions of this Part of this Schedule (not being the office of a member who was appointed by the county borough of Gateshead in the year 1969 or of any member appointed to fill a casual vacancy occurring in the office of that member) or under this paragraph shall be filled by the appointment on or before the date on which the vacancy will occur of a person to fill the vacancy and that appointment shall be made—
(a)where the vacancy arises in the office of a member appointed by the council of the county borough of Gateshead, by the council of the county borough of Tynemouth,
(b)where the vacancy arises in the office of a member appointed by the council of the county borough of South Shields, by the council of the county borough of Gateshead, and
(c)where the vacancy arises in the office of a member appointed by the council of the county borough of Tynemouth, by the council of the county borough of South Shields,
and every member so appointed shall come into office on the day on which the vacancy he is to fill will occur and shall hold office for a period of two years from that date.