Definition of a 4bet
The 4 bet is the fourth bet that can be made at a street given. You can 4 bet preflop, at the flop, at the turn, or on the river. Paying our blind at the table is said to be like a limp or a call (the 1st bet). In classic aggressive games, there will often be a raise (2 bet) 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 such as KK or AA, he will probably want to 4 bet, so make a fourth bet.
Classic example of a 4bet
A player decides to make a limp (pay his blind ) in the UTG position, therefore in first position. A player on 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 hand sees no other choice but to make a 4 bet at 25 big blinds. Unless you're in a really aggressive game, the 4 bet is often a sign of a very strong hand.