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.
Carboy 1: Quarter Century Feat Barleywine (Brewed 11/6/2014)
Carboy 2: Empty
Keg 1: The 4 Horsemen (of the Hopocalypse) DIPA - Simcoe, Amarillo, Mosaic, and Cascade
Keg 2: Beärzen Festbier
Keg 3: Saint Theakstons - PT Clone
Keg 4: Votex IPA (Citra + Centennial, bursted and dry hopped)