Step 1: Divide the number into separate digits:
8391 => 8 3 9 1
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
8 to 3 = 5
3 to 9 = 6
9 to 1 = 8
1 to 8 = 7
Step 3: Combine the filters to a string:
5 6 8 7 = 5687