I just go for overcalls. UTG waking up on the river means one of five things: 1) He has a king and figures the river counterfeited a lower two pair of yours. 2) A fourth flush card didn't come so now he's value-betting a random hand (pocket pair, a king, etc etc). 3) He's trying to represent a four on a pure bluff or semi bluff. 4) He actually has a four. 5) He has the world's worst-played baby flush but doesn't want to try for a check raise and wimped out on the turn.

He's only calling your raise here if he's on a king, some random pair, a four or a baby flush. I don't really know if we're ahead 55 % of the time that he calls - and even if we are, it seems the button will overcall a lot more often if we don't raise it.