What are the 4 diget combanations for the numbers 0-9?

Let the digit be x x x x

The first space can be filled in 9 ways as 0 is excluded. The second space can be filled in 9 ways as 0 is included. The third in 8 ways and the fourth in 7 ways.

Total number of permutations (assuming all of the digits are different) is 9 x 9 x 8 x 7 = 4,536.

If you assume the digits can repeat, the number of permutations become 9 x 10 x 10 x 10 = 9,000.