Step 1: Divide the number into separate digits:
6129 => 6 1 2 9
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
6 to 1 = 5
1 to 2 = 1
2 to 9 = 7
9 to 6 = 3
Step 3: Combine the filters to a string:
5 1 7 3 = 5173