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: 7 Day ESB, Lambastard (sour experiment with lambic dregs), Saison with Anna HF Dregs, Spontansaison.
Keg 1: Vortex the 5th (Citra, Nelson, Columbus)
Keg 2: Bearzen Marzen
Keg 3: ESB with Tea
Keg 4: Nobel Hopflogger (IPL with nobel hops)