How many different four-digit number combinations are possible from the numbers 0-9?

Answer 1

number of different 4 digit number
= 9x10x10x10 = 9000

(we cannot start with a zero, thus only 9 choices for the lead number)

Answer 2

In my answer, I assumed that you can repeat a number

If each can be used only once, then
number of ways = 9x9x8x7 or 4536