Just for reference if anyone is looking for this down the road, I put a 6 inch collar and can fit 4 kegs, albeit snugly. Still in the works, but its looking like it will turn out awesome!
Edit: This is 6 inches with a coffin keezer build, It would be difficult to do 4 with the shanks and taps through the collar.
Keg 1: Crunchberry blonde
Keg 2: Pumpkin Stout
Keg 3: Bearzen Marzen 2.0
Keg 4: Helles Keller
Keg 5: Sauerbock - bock that went sour in the barrel!