Step 1: Divide the number into separate digits:
1559 => 1 5 5 9
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
1 to 5 = 4
5 to 5 = 0
5 to 9 = 4
9 to 1 = 8
Step 3: Combine the filters to a string:
4 0 4 8 = 4048