The USAF policy passed several years ago was that "no GMO's should continue past their first tour (ie. only a GMO for 4 years max). The reason for this is that there used to be many GMO's who would not go back to residency because they could "skate" along in the military system and never have to improve their medical knowledge or get residency trained.
Many of these career GMO's were somewhat "inefficient" and used to rely on their non-expertise as a non-residency trained physician and not have to see any tough cases or referred out everything claiming they couldn't handle the case. Also, once they reached Lt Col or Colonel, they were essentially filling administrative roles and really didn't even perform any patient care. The USAF medical system finally caught on and realized this wasn't a good practice to have GMO's who wouldn't ever get board certified b/c that is what the "standard" is in civilian practice.
Funny thing is, I have seen "waivers" being given to GMO's who extend beyond 4 years (mostly for overseas assignments). Actually, I was forwarded an email 2 months ago from the RAM here stating they should be reminded that GMO's can "apply for this waiver" to extend beyond their 4 year tour.