A 4-bet is simply when a 3-bet is re-raised. Pre-flop, this happens after an initial raise and a re-raise and, as is often the case with a 3-bet, tends to be referred to as ‘pushing’ when the player’s 4-betting involves committing their entire stack. Note that in the Pre-flop stage the Big Blind represents the first bet, which in turn means that the first action (raise) is the second bet, a subsequent (re)raise is a 3-bet and, finally, a further raise is a 4-bet.
Whether to 4-bet can be a difficult decision, not least because doing so can lead to full stacks being in the middle. Consequently, we should try to take into account, when possible, the opposition’s likely 3-betting range. The tighter their 3-betting strategy, the tighter our 4-betting should be.