W E L C O M E to the Forum spaceboy!
The Smoke Generator is the same for the 4 and 6 rack. 3 bisquettes per hour
The heating element is the same too.
As far as capacity goes, it depends on what you are cooking and how hearty the appetites are.
For pulled pork (pork butt aka pork shoulder aka boston butt), they run about 6-8 pounds each, so you can get ~32 lbs of raw pork in a 4 rack. Figure 50% loss due to cooking, that leaves you with ~16 lbs of pulled pork. That should feed your crowd with leftovers, unless they (all) eat like me...

For ribs, I can get 1 rack of baby backs per Bradley rack, but if you have extra racks you can invert one over the other to get 8 racks into a 4 rack smoker.
Having already stated that the heating elements are the same, putting ~48 lbs of pork in a six rack smoker would work, but it would take a LONG time, probably 36 hours to cook. Same goes for 16 racks of baby backs.
I have an OBS - 4 rack.