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.

A Four-Fold Accumulator is one bet made up of four selections in different events. If they all win, so do you. But if any of the selections lose, your bet does too.