Step 1: Divide the number into separate digits:
4313 => 4 3 1 3
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
4 to 3 = 1
3 to 1 = 2
1 to 3 = 2
3 to 4 = 1
Step 3: Combine the filters to a string:
1 2 2 1 = 1221