Step 1: Divide the number into separate digits:
6640 => 6 6 4 0
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
6 to 6 = 0
6 to 4 = 2
4 to 0 = 4
0 to 6 = 6
Step 3: Combine the filters to a string:
0 2 4 6 = 0246