4If any person while under supervision, or after his recall to a Borstal institution, as aforesaid, is sentenced by a court in any part of Great Britain to corrective training or Borstal training or to penal servitude or detention in a Borstal institution, his original sentence of Borstal training shall cease to have effect; and if any such person is so sentenced to imprisonment, any period for which he is imprisoned under that sentence shall count as part of the period for which he is liable to detention in a Borstal institution under his original sentence,