An online casino 400% deposit bonus is an enticing promotion that significantly boosts a player's initial bankroll. When a player makes their first deposit, the casino matches it by 400%, effectively quadrupling the amount available for gaming. For example, a $100 deposit would yield an additional $400 in bonus funds, allowing for more extensive play and increased chances of winning. However, players should be aware of the associated wagering requirements and terms, ensuring they understand how to maximize their bonus while enjoying a thrilling gaming experience.