AA/KK will be 5 betting pretty much all the time to try stack an UTG 4bet which is repping a pretty strong hand, AK+/QQ+. I think its also possible he's 4betting light against a wide button 3bet but we don't know the hero's image.
When the hero flat the 4bet, we can eliminate the junk out of a polarised 3-bet range like 10-5s etc, with the top of his range being AK, JJ-QQ , and suited connectors and the bottom end. All small pocket pairs are folding (which I think would have flatted the raise to set farm) as we have pretty bad odds here.
When the flop comes 7d3d4h AK is going to fold most of the time against a half pot bet, and with the pot this large floating is a pretty bad idea, unless you think villain is 4betting light, as he is going to double barrel the turn with overpairs and his strong aces. I think this narrows the heroes range to TT, JJ, QQ and any two diamond suited connectors. However, as he calls the river with no flush draw possible, I think he must have flopped a pair or a straight on the flop - 56 of any suit, 67 of any suit. The only diamond hand he could have here is 56 of diamonds flopping the nuts which I think he will flat all the time induce bluffs - however I think 56 and 67 (now 2 pair) are going to reraise the turn to try to get the villain pot commited, stacking against overpairs. Therefore it seems to me hero is playing pot control with an overpair - I think either jacks or queens but I think he might have 5bet queens preflop as hero could have 4bet light, so because he flatted I say pocket jacks. It's so hard to determine a hand without reads on the players