What about those who have submitted their M4 times to M1? I mean, yeah, it'd be rare, but I'm pretty sure most people who've submitted regularly to the SA2 charts have figured that M1 and M4 essentially go under the same rules, so that'd technically invalidate those stats (though it'd be impossible to tell which stats are and aren't valid at that point). Only ones that really would have a difference would be Knuckles and Rouge, but technically you could count the M1s done in a single segment as M4 times, because there aren't any checkpoints in their levels. The only real difference being the timer and the ability to segment and nothing more, which is a reason I really don't like this proposal (not to mention that the only way to know for sure is if you get video proof of the stat or a picture, to make matters worse).
I also would like to point out that TECHNICALLY, M4 times can count and fall under M1 anyways, as you can follow the same rules in M1, only difference being the timer and just being a different mission (with the same layout). In contrast, M1s have checkpoints (which obviously can be ignored but still would make it invalid as an M4 submission). Say, for instance, some smart allec comes in and beats his M1 time as M4 and realizes this. He might submit the stat for both M1 and M4, causing duplicate stats. People don't tend to read rules and look through holes within them, and this is likely one of them. Then they may be confused as to why their stat got removed. I believe that this may cause a pretty big mess, but, hey, if you guys can figure it out, by all means go for it.