Can you cut into cast iron soil pipe with plastic soil pipe?

You can put up to a 2 inch pipe into a 4 inch. If you are putting 4 inch to 4 inch, either take out a section of the cast and put the PVC in or cut out a section and connect the PVC to the cast with rubber sleeves meant for this. You can use a saddle clamp for the 2 inch. It is a half round piece of PVC with a 2 inch connection in it. It is held on with two U bolts that go around the cast pipe. Put it where you need it and mark the 2 inch hole on the cast. Drill a series of 1/8 inch holes around this circle. when you get them all drilled, you can break out the plug. Try to keep it from falling into the cast pipe. Put the saddle clamp on and connect the PVC pipe as needed. Seal the saddle clamp to the cast pipe with plumbers putty.