As one who served a minority enlistment, although long ago, I suspect you may be confusing terms of service with net service for pay purposes. A minority enlistment is entered into at any time prior to a boy's 18th birthday and expires when he reaches age 21 and is no longer a minor. So, while net service could total four full years, service toward a federal pension could be little as three years and a day. Data on the reverse of one's discharge certificate should specify net (or in some cases total) service for pay purposes.
-- Fran