You can't reduce the size of the clean out.
I am putting a wet bar in my basement and I was wondering if it's OK to tap into the 4" cleanout in the main waste stack in the basement. There is a 4" cap (threaded) on the cleanout -- can I remove the cap and run a threaded 4" pipe (on one end) out of it and reduce it down to 1.5" for the wet bar drain? I'll vent the wet bar appropriately.
If I can - do I need to provide another cleanout in it's place?
But could I put in a 4 x 1 1/2 tee and add a 4" cleanout above it? Then use the 1 1/2 tee for my fixture drain?
Practically, yes. Legally, and properly, No. You at least need a proper vent to do it legally, and the fitting you would need is so specailized that very few plumbing wholesalers would even have it.
so it sounds like my best option is to just cut in a new tee in the main stack below the cleanout? Since the stack is PVC, that's not a terribly big deal.