So here is my problem. As you can see from the photos, my existing stack is cast iron. I need to tie a new 4" branch into from a new bathroom.

My problem is that the new branch is lower than the current 4" PVC branch from the old bathroom. So I can't tie into the old hub. The old vertical WYE on against the wall is a 4x4x2. So I have to change that WYE in either case.

So inclined to replace all off the cast iron with PVC up until the house trap as shown in the photos. My 2nd photo includes my suggested layout and use of new PVC fittings.

If I do change all the CI to PVC, then I guess that I have the option to lower all the fittings and bring the new branch in right under the stack, instead of extending it to the WYE.

I would appreciate comments and suggestions.

How hard would the CI removal ?