Step 1: Divide the number into separate digits:
9599 => 9 5 9 9
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
9 to 5 = 4
5 to 9 = 4
9 to 9 = 0
9 to 9 = 0
Step 3: Combine the filters to a string:
4 4 0 0 = 4400