Question 1013363
There are 256 combinations, 4^4.
You can start with 0000 then 0111 or 0888 or 0999.
You have 4 ways to choose the first and second and third and fourth.
If each digit must be used, then there are 4 ways to choose the first, 3 the second, 2 the third, and 1 the fourth, or 24 such combinations.