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: Pils
Keg 2: Bearzen Marzen
Keg 3: Tart Saison
Keg 4: Barrel aged Belgian Dark Strong