4-bet
A 4-bet is a raise over a 3-bet before the flop: the fourth bet, counting the blinds as the first.
At 100 big blinds a 4-bet commits a large part of the stack, so the range is narrow: the very best hands, and a few bluffs that hold an ace or a king.
Related terms
Practise decisions like this in the trainer: ten hands at a time, with the answer and the reason.