Step 1: Divide the number into separate digits:
5594 => 5 5 9 4
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
5 to 5 = 0
5 to 9 = 4
9 to 4 = 5
4 to 5 = 1
Step 3: Combine the filters to a string:
0 4 5 1 = 0451