Yes Virginia , there is a M-4-235.
Sorry I took soooooo long to reply to your post. Yellowmaule is right, There is such a thing as a M-4-235 . Amazing little airplane. She was created around 1979-1980. It was Done under a “one time “ STC. The STC was never sold and it’s know to be the only one in existence. ( unless there is an experimental somewhere) When I figure out how to post pictures , I’ll show you what she looks like and give you more info. As for now , the only pic is the one to the left. Yes, that’s sunlight on the bottom side of the wing.

Well between the FAA field approval process being next to impossible, my limited amount of time to deal with it, and the lack of O-540s on the market, I dropped the idea of converting the M4-220 into a -235. I chickened out and ordered a Continental IO-360 for it. Any recommendations for a propeller?

you chicken.
i'm sure we could fit a TIO540-J2BD on there.