I recently purchased a home built (I'm guessing) in the early 1900's. I noticed a leak in my basement coming from the old 4 inch cast iron soil stack running from my second floor bathroom to the basement. There is water dripping down the pipe to the basement. Three plumbers have come out to diagnose the problem. The first plumber removed the commode and cut a bigger hole in the floor to try and find where the pipes begin and end. There was also a hole cut in the floor on the other side of the wall (in the closet in the hall around the corner from the bathroom) to follow the pipes from the bathroom leading to the soil stack which then travels down through my kitchen/dining room wall to the basement. There is a lot of deterioration in my pipe at the second floor level (several holes are visible) but that is not where the leak is. Several of the plumbers believe the cast iron is cracked in the kitchen/dining room wall. The entire pipe needs to be replaced/upgraded. The plumber is thinking of putting 3 inch pvc instead of 4 inch because the 4 inch (fittings-I'm not sure?) will be a difficult fit. Is that okay to do?