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.
Fermenting: Quarter Century Feat Barleywine, Lambastard (sour experiment with lambic dregs), Simcoe Session, Nobel Hopflogger (Nobel IPL)
Keg 1: Vortex the third (Citra & Galaxy IPA)
Keg 2: Schwabisch Dunkel
Keg 3: Bearweis'n Festbier - carbing and lagering
Keg 4: SMASH ESB - Marris EKG