Step 1: Divide the number into separate digits:
9958 => 9 9 5 8
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
9 to 9 = 0
9 to 5 = 4
5 to 8 = 3
8 to 9 = 1
Step 3: Combine the filters to a string:
0 4 3 1 = 0431