This is such an uncommon situation that metagame (5-betting to stop him from cold 4-betting you) isn't really worth thinking about. 5-bet with AJo and not AK if you think he is out of line...
I agree with kdjohnson. It is a common 4b spot. Especially this deep. It is a good time to 5b for that reason of metagame. Who has to fear who. Who gets to be the aggressor gets the initiative and he has an advantage.
And the alternative of 3betting light is a good alternative. We know that he is less likely to 4b bluff in the future, because he has just 4b us.
And then the level game starts. So it is important to know your opponent. If he is good he might think like this: he knows, that you know, that he is less likely to 4b bluff. And therefor he might keep on 4betting you light. Therefor I favor 5betting here. Also because stack sizes are good for it.
I do not like 5betting AJ here. Because sometimes we will get owned by AK and other times we flop TP with the J and are against QQ. So that means reversed implied odds. I would rather 5b a hand like 76s and implied odds. But I could be wrong on this...
