There's a cleanout plug on this pipe that I simply can't get loose. I've tried liquid wrench, PB blaster, heating up the area with a torch, everything I can think of. All that's really left is to go after the plug with a drill and a saw and just pull it apart. However, none of my local hardware stores HAVE any 4 inch cast iron, all they have is PVC. Can I put a PVC cleanout plug in a Cast Iron cleanout?
Yes, it should be no problem. Just use teflon tape on the threads to seal it.
I would worry though if you can't get the plug out, that the threads in the pipe won't be any good (or you'll destroy them in the process of attempting to get the plug out). If that happens, you'll need to re-thread the pipe. You should be able to rent a tool for this at your local tool rental place.
You don't mention what the pipe is, so just be prepared that you may not be able to use it until it's repaired, if you do have a problem with the threads.
Another option would be to cut the pipe above and below the cleanout, and install a PVC Y-fitting with a cleanout cap using no-hub couplings like these:
I would think that drilling and/or sawing out the old cast iron cap is going to be a giant pain in the rear, and will be likely to damage the cleanout fitting to the point where it needs to be replaced anyway.
Make sure your cast iron stack is supported above before you cut it.
Hole saw out the center and cut a pie shaped piece out with a saws all all the way to the threads but not into the threads, then with a hammer and punch knock out the pie shaped piece and remove the rest with channel lock pliers. Do it every day as a professional plumber. And if the threads are different stop at a local plumbing outfit and get a lead plug.
Another options to the PVC one, in case the threads are damanged or the thread count doesn't match up, would be to use an expansion plug:
(I couldn't get the right size for mine at the local store, so I had to also cut down a rubber coupler to make it a closer match to the inner diameter)