Answer
9000
Work Step by Step
We can put 9 different digits in the first place (all digits, excluding zero).
We can put 10 different digits in the second place (all digits).
We can put 10 different digits in the third place (all digits).
We can put 10 different digits in the fourth place (all digits).
The number of four-digit numbers is the variation of these four:
$9\times 10\times 10\times 10=9000$
