4. An officer to whom paragraph 3 applies may, instead of retaining that officer’s entitlement to that officer’s ordinary pension under regulation B1 of the 1987 Regulations or, as the case may be, that officer’s deferred pension under regulation B5 of those Regulations, elect to relinquish that officer’s entitlement to the ordinary or deferred pension and to reckon as pensionable service for the purposes of these Regulations the pensionable service the officer was entitled to reckon under the 1987 Regulations at the time of that officer’s retirement with an entitlement to the ordinary or deferred pension.