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?
There is not problem with tapping into the cleanout. Leave yourself a cleanout in the same spot. Your new fixtures should be vented.