$\_\; \_\;\_\; \_$
Since, number of digits is $4$ the number cannot start with $0.$
At first place all number can appear except $0.$ So, number of possibilities $= 9.$
$9\; \_\;\_\; \_$
Since, repetition is not allowed, second place can be occupied by $8$ different numbers. But here $0$ may appear so total possibilities is $9$.
$9\times9\;\_\; \_$
Similarly, remaining two places can be filled in
$9\times9\times8\times7= 4536.$