I need to add a trap to my cast iron drain in my basement.

I'm going to remove my 4" cleanout and run the trap into that fitting.

Here's the difficulty:
My cleanout plug is threaded into a bushing held into the bell end of
a cast fitting. The threaded cleanout is a little shy of 4" (about 3
15/16) I bought a 4" threaded male PVC fitting but it is about 4 1/4.

Do they make a threaded PVC fitting that will go into a 4" female
steel?

Should I just remove the bushing and get the rubber boot that will let
me go from the bell end of the cast down to 4" PVC?