Any person committed under the provisions of § 19.2-311 shall be eligible for release at the discretion of the Parole Board upon certification by the Director of the Department of Corrections that the person has successfully completed the program established pursuant to § 53.1-63 and a determination that he has demonstrated that such release is compatible with the interests of society and of such person and his successful rehabilitation to that extent. The Department and Parole Board shall make continuous evaluation of his progress to determine his readiness for release. All such persons, in any event, shall be released after four years’ confinement. Any person committed under § 19.2-311 who was convicted of a misdemeanor and is determined to be unsuitable for the program established pursuant to § 53.1-63 shall be released after one year of confinement or the maximum confinement for the misdemeanor whichever is less.