Step 1: Divide the number into separate digits:
7157 => 7 1 5 7
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
7 to 1 = 6
1 to 5 = 4
5 to 7 = 2
7 to 7 = 0
Step 3: Combine the filters to a string:
6 4 2 0 = 6420