4-Bet

A simple way to explain a 4-bet is referring to it as re-raising a 3-bet, usually in a preflop betting round.  

In this scenario, the blinds are considered the first bet, the second bet is the initial raise, the re-raise is the third bet, and the bet re-raising the 3-bet is the 4-bet. If someone makes a 3-bet and you decide to raise it again, your bet is called a 4-bet.

Poker players don't usually 4-bet after the flop, so you can assume that this term means the action before the flop.

For example, you are playing in a $1/$2cash game, and you have AKs in the Big Blind. Everyone folds to Hijack who raises to $6, Cutoff 3-bets to $18, and since you are holding one of the best possible hands, you decide to re-raise to $45, thus making a 4-bet.

This can be a powerful move when you have a strong hand and want to build the pot or a great option to bluff your opponents if they are 3-betting too much.