Just a quick question.

You are much better with a 4 inch. You will need a 2X6 wall but you can usually hide it inside a closet. 4" is 30% larger and with a low flow toilet would help to keep it open.

Depending how much space you have and how many plumbing fixtures you plan on connecting to this waste pipe. 3" will easily carry on bathroom group - then connect it to 4" main sewer somewhere along the way. As stated in response #2, it is easier to work with 3" pipe in tight places, like between joists, 2" x 4" walls, etc. Also, if you are fighting slope, 4" pipe requires only 1/8" per foot slope while 3" pipe requires 1/4" per foot of slope.

Its on a slab that still has to be poured on lower floor. So I geuss 4 is the best size.

Its on a slab that still has to be poured on lower floor. So I geuss 4 is the best size.

My code, (SPC) mandates a 4" house main and 4" toilet branches under the slab. However, on as two story job we can reduce the toilet branch to 3" in then second story , Good luck, Tom