Wow, that's a doozie. I'd have to agree that KJ seems likely. I'm not sure what I'd do or what the right thing to do here is. Just keep in mind, even if you're right on your guess of KJ, you've got close to a 25% chance of winning on the river (although if you were near certain he had it, it goes without saying that you would not have odds
to call, but I'm guessing we're not near certain). Got any reads on the guy?
It's also a possibility that he hit a set on the flop and maybe either has a read that you wouldn't have raised 4 BBs with KJ pre-flop or is just hoping that you don't have that hand, or didn't even notice the straight possibility because he's in love with his set. All I know is that, screw odds, 100% of the time I call this my opponent will turn over KJ.