If you allow zero to be in the first positions (0000 and 0001, etc) then there are 10,000 combinations (0000 to 9999). If zero can not be in te first position, remove 0000 to 0999 (1,000 combinations), so there are 9,000 combinations (1000 to 9999).
In base ten, there are 9000 four digit number, the numbers are from 1000 to 9999. If you allow leading zeros, then there are 10000 numbers. [0000 - 9999]
The formula is 9*9*9*9* * * * * *The above answer is wrong on two counts: First, there are ten digits to chose from, not nine. Second, the answer is in terms of permutations ie 1234 being different...