Step 1: Divide the number into separate digits:
3988 => 3 9 8 8
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
3 to 9 = 6
9 to 8 = 1
8 to 8 = 0
8 to 3 = 5
Step 3: Combine the filters to a string:
6 1 0 5 = 6105