A "four-digit" number is any numerical value that consists of exactly four digits, ranging from 0000 to 9999. This means it can include numbers like 1234, 0001, or 9999. Four-digit numbers are commonly used in various contexts, such as PIN codes, identification numbers, and years in the calendar. In mathematics, four-digit numbers can be classified as either even or odd, depending on their last digit. They can also be categorized as prime or composite numbers. Understanding four-digit numbers is essential for tasks like data entry, coding, and financial transactions, where precision is crucial.