The term 3-bet most commonly refers to the first re-raise before the flop, although the term also refers to the first re-raise after the flop. (This article focuses exclusively on preflop 3-betting.) Subsequent raises are referred to as a 4-bet, 5-bet, etc.
A 4-bet is the second re-raise in a betting round. 4-bets are significantly more common before the flop, but they can happen after the flop as well. For example, suppose you raise preflop and a player behind you re-raises (aka 3-bets). If the action gets back to you and you re-raise again, that's a 4-bet.
