Step 1: Divide the number into separate digits:
6609 => 6 6 0 9
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
6 to 6 = 0
6 to 0 = 6
0 to 9 = 9
9 to 6 = 3
Step 3: Combine the filters to a string:
0 6 9 3 = 0693