[Grease]

I don't think MP3 has a flush if he knows how to play.

If he had a flush, he would C/R the flop and trap everyone with his big hand. I think you're looking at two pair (maybe AT or A4?).

I like a turn raise, but I don't mind the calldown, since he could 3-bet if he has a flush. Although you're compelled to call since you have 4 outs in this fairly large pot.

If you raise and he 3-bets, can you fold the river UI?

Yeah, I like a turn raise.