**First digit 1 of 9 (excludes 0 thus 1,2,3,4,5,6,7,8,9 = 9 options.
**

Second digit 1 of 9 (includes 0) thus 0 plus 8 of the remaining 9 = 9 options.

Third digit 1 of 8 total of 10 digits minus the 2 already used = 8 options.

Fourth digit 1 of 7 total of 10 digits minus the 3 already used = 7 options.

9*9*8*7=4,536 different numbers.