There are 10 digits from 0 to 9
When you say 4 digit number that means it can't start with zero, then
1) there are 9 ways to chose the 1st digit (excluding zero)
2) there are only 1 ways to chose the 2nd digit (only 8))
3) there are 10 ways to chose the 3rd digit
4) there are 10 ways to chose the 4th digit
TOTAL NUMBER OF WAYS: 9 x 1 x 10 x 10 = 900