Here's the reg:
http://comptroller.defense.gov/fmr/current/07a/07a_01.pdf page 11 and 44
010103. Active Enlisted and/or Warrant Officer Service for Members in Basic Pay Grades O-1E, O-2E, or O-3E
A. Service Counted
* 1. General. Commissioned officers with over 4 years of prior active service as an enlisted member, warrant officer, or combined service in both grades are entitled to count such service for purposes of computing basic pay for longevity purposes. Such prior service includes all active service, in either the Regular or Reserve Component or both (i.e., active duty for training in enlisted or warrant officer status, annual Reserve training duty, and full-time National Guard duty). Service on active duty or active and inactive duty for training for at least 4 years and 1 day satisfies the over 4 years of service requirement under this section. See subparagraph 010102.A.8 and applicable notes to Tables 1-7 and 1-8 for student entitlement to basic pay for longevity purposes while attending the USUHS.
* 2. Creditable Prior Enlisted Service. Commissioned officers in pay grades O-1, O-2, or O-3 who are credited with over 4 years (i.e., at least 4 years and 1 day) of prior active service as an enlisted member are entitled to the special rate of basic pay for pay grade O-1E, O-2E, or O-3E.