A: Yes it has a usage range of 3.8"–4.06" ID.
A: I don't see why not. I bought this for a cover for my main clean out because it was recommended by my plumber. Just make sure that the wider end is wider than your toilet flange so that there is no chance of it dropping in when tightening it down
A: I used it as a plug for the clean-out of the bathroom stack (the old one was actually a cork). A plumber put one just like it in the kitchen stack clean-out. They work. As far as what an inspector would say in the process of selling the house, I can't say.
A: This is an approved plug but you will need to check with your local municipality to ensure it is approved in your area. These are typically used for testing or temporary plugging of pipe and not a permeant plug.
A: Yes it does.