Step 1: Divide the number into separate digits:
9404 => 9 4 0 4
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
9 to 4 = 5
4 to 0 = 4
0 to 4 = 4
4 to 9 = 5
Step 3: Combine the filters to a string:
5 4 4 5 = 5445