A "Four-Number Combination" refers to a sequence of four digits used for various purposes, such as unlocking a safe or accessing a secure system. Each digit can range from 0 to 9, allowing for a total of 10,000 possible combinations (from 0000 to 9999). This makes it essential to choose a combination that is not easily guessable to enhance security. In many cases, users are encouraged to avoid common patterns, such as 1234 or 0000, to prevent unauthorized access. Additionally, some systems may implement rules that require a mix of numbers or prohibit repeated digits, further increasing the complexity of the combination.