A 400% match deposit bonus is an enticing promotional offer often used by online casinos and sportsbooks to attract new players. When a player makes their initial deposit, the casino matches it by 400%, significantly boosting the player's bankroll. For example, a $100 deposit would result in a total of $500 to play with, including the bonus. This generous offer allows players to explore a wider range of games or bets, enhancing their gaming experience. However, it's essential to read the terms and conditions, as wagering requirements may apply before any winnings can be withdrawn.