Step 1: Divide the number into separate digits:
4599 => 4 5 9 9
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
4 to 5 = 1
5 to 9 = 4
9 to 9 = 0
9 to 4 = 5
Step 3: Combine the filters to a string:
1 4 0 5 = 1405