Step 1: Divide the number into separate digits:
3966 => 3 9 6 6
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
3 to 9 = 6
9 to 6 = 3
6 to 6 = 0
6 to 3 = 3
Step 3: Combine the filters to a string:
6 3 0 3 = 6303