4-bet AK against a light 3-bet isolation range. It's not really +EV until $200NL+. It happens at $100, but rarely. 4-betting AK is a realistic and legitimate play. It's VERY tricky however. You need to have the correct equity based on a host of factors.

At lower limits I would 4-bet AK against a maniac 3-bettor, because he'll stack off with bullshit.