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If you're allowed to repeat digits, then there are (4 x 4 x 4 x 4) = 256 .
If you're not allowed to repeat digits, then there are (4 x 3 x 2 x 1) = 24 .
There are 12 such numbers.
9,000
4! = 24 ■
There are 7200 such numbers.
3024