There has been a poker mantra going around that the 4th bet preflop is Aces. While it's not always true, you wouldn't have thousands of people reciting it if there wasn't a solid reason behind it.

♠ Catching a Squeezer
One situation where you'll find a fourth bet going into the pot pre-flop is when the someone doesn't believe the "squeezer". One example might be: Under The Gun folds, next player raises. The next player calls the raise, and person in the CutOff squeezes by raising both of them. After all, he's squeezed successfully twice already since he joined the table.

The button raises the the man in the Cutoff. Maybe because the CutOff has been squeezing every chance he gets. Maybe it's because the button has Aces or Kings. Who's really sure? But, this is definitely a case where someone could be getting a fourth bet into the pot preflop.

♠ The Stealer vs the Blinds
One of the more popular moves in poker involves raising when you are the first to enter the pot in Late Position. Often, the blinds will not have strong enough holdings to put up a fight, and they'll fold. This allows someone to effectively steal the blinds.

Of course this play can be overused, and eventually the players in the blinds will begin to fight back, often by re-raising. That's the third bet of the hand.

At this point, the stealer may decide to call, and play the hand out. Or, they may decide to send a message that "this time I'm not stealing", and put a fourth bet into the pot.

♠ I Got Aces (or Kings)
With the mantra of "Never fold KK pre for 100bb or less", expect big hands to be getting it in if they can. Which, is something everyone already knew.

In the end, the is not always Aces. But, it's seldom anything but a very strong hand.