Step 1: Divide the number into separate digits:
4395 => 4 3 9 5
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
4 to 3 = 1
3 to 9 = 6
9 to 5 = 4
5 to 4 = 1
Step 3: Combine the filters to a string:
1 6 4 1 = 1641