Step 1: Divide the number into separate digits:
6291 => 6 2 9 1
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
6 to 2 = 4
2 to 9 = 7
9 to 1 = 8
1 to 6 = 5
Step 3: Combine the filters to a string:
4 7 8 5 = 4785