Step 1: Divide the number into separate digits:
3917 => 3 9 1 7
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
3 to 9 = 6
9 to 1 = 8
1 to 7 = 6
7 to 3 = 4
Step 3: Combine the filters to a string:
6 8 6 4 = 6864