law: 5. Liability to complete interrupted service.-
  If any qualified person serving in the Armed Forces of the Union ceases to serve therein before he has completed four years of such service, he shall, unless he has ceased to be liable under this Act to be called up for national service, be liable to be called up to serve for such term as will, together with the service completed by him, be equivalent to the term of service for which persons are liable to serve under this Act.