I am remodeling my guest bathroom and just decided to replace the existing cast plumbing with ABS. The existing plumbing runs (2) 4" drain pipes into a 4 x 4 x 4 x 4 double sanitary tee ((2) in, (1) cleanout and (1) out). I cannot find this fitting in ABS, would a 3" drain pipe be sufficient to run a toilet and a shower? I could then run 4 x 3 closet bends, and a 4 x 4 x 3 x 3 double sanitary tee.

Thanks

Yes, 3" is sufficient for toilet and shower. Are toilet and shower vented?