Step 1: Divide the number into separate digits:
6661 => 6 6 6 1
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
6 to 6 = 0
6 to 6 = 0
6 to 1 = 5
1 to 6 = 5
Step 3: Combine the filters to a string:
0 0 5 5 = 0055