Probably fold to the cold 4-bet. UTG opened, you 3-bet from MP, and the BB cold 4-bet you. I don't think people are going to try to level you here into thinking that it's such a strong line, and make you fold premium hands. I think you're just being pretty results-oriented. There could be some merit in flatting since the UTG fish came along, and you're getting a very good price. Even then, you will 100% have RIO against his value range, unless you flop an A against his KK. I don't think people are bluffing
here often at all.
And sometimes fish will min 4-bet/5-bet premium hands, especially AA. I once tank-folded KK to a min 5-bet and was correct.
Think about it. His line looks so strong, and he might just be trying to induce with AA. He might be thinking he wants to get value from QQ/JJ (which would almost always fold to a large 4-bet), or get AK to spaz.
I don't get the hype about Pokerstars being great players, really. I've watched a bunch of Nanonoko's 25NL-100NL Zoom, and even there they aren't that strong. Sure, they might be the best site in terms of difficulty overall, but not by how much people claim their difficulty to be. I've been looking at a lot of your hand history posts too, which further back up my previous beliefs. They just don't play their hands very well lol.