As the others have said, I don;t know how you fols KK here. If you're going to fold on a 3 undercard flop (3 undercards which he can't possibly have) then you're effectively set mining with KK, which is horrible. The only reason to flat his 4bet is if you give him credit for being decent enough to fold JJ/QQ/AK to you're 5bet, so you're trying to get at least a cbet from them. You just have to get it in with KK here and accept the cooler if he has AA.