What are all the possible combinations for the #'s 0-9?

Question

numbers can repeat
they must be 4 numbers in every group

Answer 1

That's pretty easy. start with 0000, 0001, 0002, etc., to 9999
There are 10,000 of these numbers (1-9999 and 0000).

Answer 2

Well think of it this way:

You need to fill 4 slots with 10 numbers (0-9).

Since they can be repeated you have an algebraic representation:

10*10*10*10

That is 10000