Step 1: Divide the number into separate digits:
4239 => 4 2 3 9
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
4 to 2 = 2
2 to 3 = 1
3 to 9 = 6
9 to 4 = 5
Step 3: Combine the filters to a string:
2 1 6 5 = 2165