Step 1: Divide the number into separate digits:
9491 => 9 4 9 1
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
9 to 4 = 5
4 to 9 = 5
9 to 1 = 8
1 to 9 = 8
Step 3: Combine the filters to a string:
5 5 8 8 = 5588