[Grunch]

The major reason why I think a 4bet that is not all in is usually bad is because of postflop stack sizes. You will be left in a position where you will be virtually pot stuck to any hand, and you will be hamstrung in your ability to CB. Another reason is that I think we have limited FE on the flop if we decide to CB, and if we 4bet PF with AK it must be with the intention of CBing the flop when it is wise to do so.

Consider why you 4bet preflop at all. Is it because you think you have the best hand? I doubt it. If you get a call here, it's not from AJ. Rather, a 4bet is a semi-bluff. It's a bet that you'd like to not be called, but if you do you don't mind too much because you have some equity. But where does that equity come from? If you don't hit your A or K on the flop, are you going to quit? If so, wouldn't smooth-calling preflop be better?

I believe that if you are only going to put mmore chips in postflop with AK when you improve then you're better off not trying to forcibly retain the lead preflop. (I'm not saying fold) Conversely if you do forcibly retain the lead preflop, you must CB the flop when it is strategically wise to do so, if you have chips left. But if we just re-raise not all in preflop we're going to have about a pot-sized bet left. Any CB is effectively a push because we won't be able to fold correctly if we underbet and get raised. Moreover we probably have more FE by pushing preflop than we do by potting or underbetting a rag board when the opponent is extremely likely to have an overpair to the board.

Those are basically my thoughts...