Step 1: Divide the number into separate digits:
9901 => 9 9 0 1
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 1 = 1
1 to 9 = 8
Step 3: Combine the filters to a string:
0 9 1 8 = 0918