4. An officer to whom paragraph 3 applies may, instead of retaining his entitlement to his ordinary pension under regulation B1 of the 1988 Regulations or, as the case may be, his deferred pension under regulation B5 of those Regulations, elect to relinquish his entitlement to the ordinary or deferred pension and to reckon as pensionable service for the purposes of these Regulations the pensionable service he was entitled to reckon under the 1988 Regulations at the time of his retirement with an entitlement to the ordinary or deferred pension.