Oh, isn't that a happy little question! There are 10,000 different combinations for a four-digit number because each digit can be any number from 0 to 9. Just imagine all the beautiful possibilities you can create with those numbers. Remember, there are no mistakes, just happy little accidents in the world of numbers.
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0, 1, 2, 3, 4, 5, 6, 7,.......101, 102, 103, 104, 105, 1001, 1002, 1003, 1004, 1005,........ 5001, 5002, 5003, 5004, 5005,.......9996, 9997, 9998, 9999
If it MUST be all four digits then the first thousand would just be changed to 0000, 0001, 0002, 0003 and so on.
0000, 0001, 0002, ...., 9997, 9998, 9999
If every number can be used as many times as you like, there are 104 = 10000 different combinations. If each number can only be used once, there are 9!/(9 - 4)! = 5040 combinations.
there are 10000
every number from 0000 to 9999
10000 ; This may look to be wrong but you can have 0001 to 9999 which gives you 9999 different combinations of 4 digits, plus another one for 0000.