A 4-bet is when you re-raise a 3-bet before the flop. When a player 4-bets, they are representing a huge hand.
In fact, the 4-bet used to always mean that your opponent has KK or AA, but these days it can mean quite a few different things depending on the player.
Some players use the 4-bet as a bluff in key spots.
BASICS OF 4 BETTING
In general, 4-betting should still be viewed as a very strong play, and you should tend to believe that your opponents have a strong hand in most cases.
This is especially true at the low-mid stakes. But pay attention as you will undoubtedly come across players who use the 4-bet as a bluff.
An example of a 4-bet bluff is when a player is raising a lot pre-flop, and you choose to 3-bet him. He may 4-bet you thinking that you’re just exploiting his wide opening range with your 3-bet. Basically, he may very well be 4-betting with intentions to fold to another raise.
“I believe in poker the way I believe in the American Dream. It enriches the soul, sharpens the intellect,
heals the spirit, and — when played well, nourishes the wallet.”
CORRECTLY SIZING YOUR 4 BETS
If you have over 40 big blinds and are 4-betting an opponent for value or as a bluff, size your raise 2x-2.5x their bet.
If you have 30-40 big blinds or less, it is usually just best to go all in as your 4-bet instead of sizing a specific bet.