Question 98312
There are 10 ways to choose the first digit.
9 ways to choose the second because the tenth digit is used in the first position.
8 ways to choose the third (the other two are used in positions 1 and 2) and 7 ways to choose the fourth digit (the other three have already been used).

This is a total of 10 x 9 x 8 x 7 = 5040 combinations