Step 1: Divide the number into separate digits:
8421 => 8 4 2 1
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
8 to 4 = 4
4 to 2 = 2
2 to 1 = 1
1 to 8 = 7
Step 3: Combine the filters to a string:
4 2 1 7 = 4217