How can I remove a plastic flange from a 4 inch PVC pipe?
I've seen some discussion here of fancy plumber tools (a bit too expensive for the amount of use anyone but a plumber would put them to, unless you can find them in a rental store) that apply heat and grip/pull things apart.

Other than that (which I've never seen or used, only seen mentioned), I'm prone to consider PVC joints as forever, so the stock approach when you need a new one is to cut at a point where you can install a coupling, some new pipe (in most cases), and the new fitting.