What can villain have here?

It's a 4bet pot. AK is possible, but I would expect villain would barrel it on the turn.
It more smells like a set of QQ or AA, but it's only 1 combo possible for both hands.
For a min raise it's tough to fold indeed. If I was villain I would shove with all my strong hands.
Even against a shove it's hard to fold. It's only 53c more.

Cheers,
SDK1987

Some thoughts on pre-flop:
[*] Generally I don't see click-4b IP as a bluff (certainly on 5NL where 3b is even something people do to little off) => allthough your small 3b size might induce some players... => My guess is KK+ or if it's a bad player a suited Ax (his click 4b is a great filter on you, you would shove your AKs, QQ+ so he already has the read that you don't have toppair)
[*] I would fold preflop my AQo out of position as it will be hard to win the pot if you miss and even if you hit you don't know what hand he click-4b. Might be trash but might be very strong.
[*] 3b OOP = 4x to avoid easy calls from speculative hands IP.
Postflop:
[*] Why bet on river? You played your hand mainly passive and if you're willing to call 60% pot, why not give villian a chance to bluff his KK or possibly valuebet worse 2P like suited A9, A3, A6 that might have played like this preflop?
[*] If you want to blockbet on river, you should use smaller sizing: blockbet is used to make villian call with any good pair or worse and better hands (2P or better) will raise. With your used size you indicate strength and any good player will fold Top pair only hands, call 2p and raise top 2P or sets.
[*] As played you should call but I don't expect you winning here a lot.