Full explanation — 4-Bet
A 4-bet is the fourth raise in a preflop betting sequence: an open raise (2-bet), a re-raise (3-bet), and then the 4-bet in response. A well-constructed 4-betting range combines premium value hands like QQ+ and AK with a small number of selected bluffs, such as A5s, so that the range isn't too easy to read and exploit. 4-betting is a key tool for punishing opponents who 3-bet too loosely.