Step 1: Divide the number into separate digits:
6198 => 6 1 9 8
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
6 to 1 = 5
1 to 9 = 8
9 to 8 = 1
8 to 6 = 2
Step 3: Combine the filters to a string:
5 8 1 2 = 5812