There are a total of 10,000 four-digit combinations using the numbers 0 to 9, assuming that 0 is allowed as the first digit and repetition of digits is allowed. Some of the possible combinations are 1911, 0000, 3145 and 2458.
Four-digit combinations of the numbers 0 to 9 are often used as personal identification numbers to provide security to users of debit cards. An example of these combinations may be formed by choosing a digit between 0 and 9 and then repeating this act three times. There are 10 possible outcomes each time a digit is chosen, so there are 10^4 total possible combinations. Assuming it takes a second to write each combination, it would take a little less than three hours to write down all the possible combinations.
While most of these are completely arbitrary, there are many interesting four-digit combinations of the numbers 0 to 9 which have special properties or secret meanings. Some combinations, such as 1221 or 3443, are palindromic numbers, which read the same forwards as backwards. Another example is 1337, which is used by computer hackers as a slang description of elite skill. Other examples represent years associated to famous historical events, such as 1066 being the year of the Norman conquest of England. Other four-digit combinations, such as 1984 or 2001, are associated with the titles of well-known literary works.