Step 1: Divide the number into separate digits:
6623 => 6 6 2 3
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
6 to 6 = 0
6 to 2 = 4
2 to 3 = 1
3 to 6 = 3
Step 3: Combine the filters to a string:
0 4 1 3 = 0413