Answer: 4536
Step-by-step explanation:
There are ten single digit values 0 through 9. But we cannot use 0 as the first digit, so we have 9 possible choices for the first slot.
For the second slot, we also have 9 choices because we can now pick 0.
The third slot has 8 choices.
The last slot has 7 choices.
Each time we form a new slot, the number of choices drops by 1. The exception of course is the jump from the first to second slot.
Overall, there are 9*9*8*7 = 4536 different numbers.