i see this the reverse as most here, at 50nl this is usually aces on the 4bet unless you have reason to believe otherwise. If the positions were reversed id 4bet shove with KK, but once he 4bets like that either u have the same hand or he has aa. the % of people making a 4bet with QQ, AK at 50nl is very low.

the key here is that he didn't shove, he 4bet you. its a fold. If he had shoved, you have to call. you can go on about blah blah we're exploitable doing this but really nobody is paying any attention to your 4bet folding range at 50nl