Step 1: Divide the number into separate digits:
5150 => 5 1 5 0
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
5 to 1 = 4
1 to 5 = 4
5 to 0 = 5
0 to 5 = 5
Step 3: Combine the filters to a string:
4 4 5 5 = 4455