I think you maybe miswrote here (at least I think you did): the 4 years at USUHS doesn't count towards "payback" but does count towards the multiplier when calculating retirement pay once past 20yrs.
You won't see a bump in active duty pay once you hit 20, but it's my understanding that when you retire you get credit for those 4yrs at USUHS.
I hope I'm not mistaken in that...