Step 1: Divide the number into separate digits:
4039 => 4 0 3 9
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
4 to 0 = 4
0 to 3 = 3
3 to 9 = 6
9 to 4 = 5
Step 3: Combine the filters to a string:
4 3 6 5 = 4365