Step 1: Divide the number into separate digits:
4221 => 4 2 2 1
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
4 to 2 = 2
2 to 2 = 0
2 to 1 = 1
1 to 4 = 3
Step 3: Combine the filters to a string:
2 0 1 3 = 2013