Step 1: Divide the number into separate digits:
4219 => 4 2 1 9
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
4 to 2 = 2
2 to 1 = 1
1 to 9 = 8
9 to 4 = 5
Step 3: Combine the filters to a string:
2 1 8 5 = 2185