Step 1: Divide the number into separate digits:
5118 => 5 1 1 8
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
5 to 1 = 4
1 to 1 = 0
1 to 8 = 7
8 to 5 = 3
Step 3: Combine the filters to a string:
4 0 7 3 = 4073