Definition of a 4-bet
The 4-bet is the fourth bet one can make on a given street. We can 4-bet preflop, on the flop, on the turn or on the river. Paying our blind at the table is said to be a limp or a call (the 1st bet). In classic aggressive games, there will often be a raise (2bet) to say 3 big blinds. A player in position may want to isolate that player by making a 3 bet at say 10 big blinds. And if a player in the blinds ends up with a premium hand like KK or AA, he will certainly want to 4-bet, so make a fourth bet.
Classical example of a 4-bet
A player decides to limp (pay his blind) in UTG position, so in first position. A player on the cutoff raises this bet to 3 big blinds. The player on the button decides to make a 3 bet to 10 big blinds to reduce the number of players who will see the flop. The player on the small blind, who has a pair of aces in his hand, sees no choice but to make a 4-bet to 25 big blinds. Unless you are in a really aggressive game, 4-bets are often a sign of a very strong hand.