Step 1: Divide the number into separate digits:
9951 => 9 9 5 1
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 1 = 4
1 to 9 = 8
Step 3: Combine the filters to a string:
0 4 4 8 = 0448