Obvious but tired.
3.5 bathrooms and i vent stack.
What makes it worse is 2" inch ( master bath ) PVC is taped into a 2" PVC off tyhe main 3rd floor bath, then expanded into one 4" PCC 12 inches before the exit through the roof deck.
Washer dry , powder room with shower cabinet.
1/2 bath on first floor.
Kitchen with island and double sink.
here is the 4" enlarged PVC fitting and exit.