Step 1: Divide the number into separate digits:
6157 => 6 1 5 7
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
6 to 1 = 5
1 to 5 = 4
5 to 7 = 2
7 to 6 = 1
Step 3: Combine the filters to a string:
5 4 2 1 = 5421