I'd imagine that your hand is very transparent to your opponent in this situation considering how you played pre-flop and then calling the flop bet OOP. AK is your most likely candidate in his eyes. That being said, he either thinks he can muscle you off TPTK, or he has TPTK beat. Should he have a reason based on your play to think he can push you off TPTK? If not, he can probably beat TPTK or has the same hand as you and is testing your guts. For an AKo hand, you got a dream flop. If you fold it now, how can you ever play it in the future? The entire value of AKo is TPTK on the flop. There are only 3 ways he can have aces and 1 way he can have kings, so only 4 starting hands have you beat unless you think he hood winked you preflop 4-betting a funky hand. His 4-bet bought position for him by folding the original raiser, so it is possible that he was isolating you figuring he knew your probable holdings. All this being said AKo plays horribly OOP, so those advocating 5-betting preflop, I'm with you guys.