In position I'd never more than 2x his raise with 100bb effective (which we have). OOP I think it'd be valid to 3bet to 2.8 or so.

Flatting is obviously extremely dynamic dependent. Is there 3bet history? Have you 4bet before? Does he cbet every time he 3bets? Does he c/r a lot when he 3bets? Those are the types of questions you should be asking yourself during the hand. Generally, I hate doing anything but 4betting AA either IP or OOP but if he's a 10/9 super nit with 99% fold to 4bet over a million hands I'd hesitate to do so.