9 * 9 * 8 * 7 = 4536 possible numbers
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There are nine feasible choices for the first digit (any digits ranging from 1 - 9). Once this has been chosen, nine choices for the next digit still remain (notice that the first digit of the number cannot be repeated, however, you may use 0). There are then eight available choices left for the third digit and seven available digits left for the fourth digit. Overall, the number of possible numbers is (9 * 9 * 8 * 7) possible numbers.
There are nine feasible choices for the first digit (any digits ranging from 1 - 9). Once this has been chosen, nine choices for the next digit still remain (notice that the first digit of the number cannot be repeated, however, you may use 0). There are then eight available choices left for the third digit and seven available digits left for the fourth digit. Overall, the number of possible numbers is (9 * 9 * 8 * 7) possible numbers.