Step 1: Divide the number into separate digits:
9164 => 9 1 6 4
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
9 to 1 = 8
1 to 6 = 5
6 to 4 = 2
4 to 9 = 5
Step 3: Combine the filters to a string:
8 5 2 5 = 8525