A 4-Bet in poker is another raise that follows an already existing re-raise, or 3-Bet. This occurrence is common during pre-flop round, although it can be effectively used in post-flop rounds as well.
For example, let’s say that you’re playing in a $1/$2 cash game and decide to open the pot to $6 in the BTN position only to see the Big Blind 3-bet you to $20. If you re-raise after this and make it $44, this is known as a 4-Bet.