4Where his offence term and his non-offence term are partly concurrent—
(a)section 1(1) or (2), or as the case may be those provisions as modified by section 5(2), of this Act shall apply in relation to the term which is due to expire later and shall not apply to the term which is due to expire first; and
(b)if the term due to expire later is his offence term, section 1(3) of this Act shall apply in relation to it only if the person has served such proportion of his non-offence term as would, but for sub-paragraph (a) above, entitle him to release under section 1(1) or (2), as modified by section 5(2), of this Act.