Step 1: Divide the number into separate digits:
8159 => 8 1 5 9
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
8 to 1 = 7
1 to 5 = 4
5 to 9 = 4
9 to 8 = 1
Step 3: Combine the filters to a string:
7 4 4 1 = 7441