Step 1: Divide the number into separate digits:
4999 => 4 9 9 9
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
4 to 9 = 5
9 to 9 = 0
9 to 9 = 0
9 to 4 = 5
Step 3: Combine the filters to a string:
5 0 0 5 = 5005