Step 1: Divide the number into separate digits:
9515 => 9 5 1 5
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
9 to 5 = 4
5 to 1 = 4
1 to 5 = 4
5 to 9 = 4
Step 3: Combine the filters to a string:
4 4 4 4 = 4444