Step 1: Divide the number into separate digits:
5995 => 5 9 9 5
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
5 to 9 = 4
9 to 9 = 0
9 to 5 = 4
5 to 5 = 0
Step 3: Combine the filters to a string:
4 0 4 0 = 4040