I have just about finished adding a new bath. All I have left to do is connect the drain into the main waste stack which is 4" cast iron. I have seen where people have cut into the stack and spliced in a 4" PVC Y fitting and then used rubber boots at both ends to seal. I am just a little concerned about the weight of the waste stack on the splice. I thought of tapping into the waste cleanout cap and putting a PVC Y fitting there. This would raise my cleanout cap about 10 inches higher than it already is. Any advise?