Step 1: Divide the number into separate digits:
9909 => 9 9 0 9
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
9 to 9 = 0
9 to 0 = 9
0 to 9 = 9
9 to 9 = 0
Step 3: Combine the filters to a string:
0 9 9 0 = 0990