What are the 4 digit combinations that can be made from 1239?

There are 24 permutations with four digits, assuming no repetition:

These are as follows:
{1,2,3,9} {1,2,9,3} {1,3,2,9} {1,3,9,2}
{1,9,2,3} {1,9,3,2} {2,1,3,9} {2,1,9,3}
{2,3,1,9} {2,3,9,1} {2,9,1,3} {2,9,3,1}
{3,1,2,9} {3,1,9,2} {3,2,1,9} {3,2,9,1}
{3,9,1,2} {3,9,2,1} {9,1,2,3} {9,1,3,2}
{9,2,1,3} {9,2,3,1} {9,3,1,2} {9,3,2,1}

The formula to work out permutation without repetition is n! / (n - r)!.