A 4bet is the fourth bet in a sequence, effectively a re-raise of a re-raise. Pre-flop it typically indicates a very strong hand such as AA or KK, though it can also be a polarized bluff. 4bet ranges are generally narrower than 3bet ranges. When a player 4bets they are often signaling a willingness to get all the chips in, especially in tournament play with shallower stacks.