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.
The Martingale strategy involves doubling up on losing bets and reducing winning bets by half. It essentially a strategy that promotes a loss-averse mentality that tries to improve the odds of breaking even, but also increases the chances of severe and quick losses.