4.0.1. Where his conditions of employment so provide, the person referred to in section 4 may have the days and parts of a day during which he benefited from a full-time leave without pay for a period that preceded immediately the beginning of the leave referred to in this section counted in accordance with the provisions of the retirement plan of which the person is a member respecting the redemption of a leave without pay even if he does not hold pensionable employment and except if that leave has been otherwise credited under his retirement plan.