Step 1: Divide the number into separate digits:
9964 => 9 9 6 4
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
9 to 9 = 0
9 to 6 = 3
6 to 4 = 2
4 to 9 = 5
Step 3: Combine the filters to a string:
0 3 2 5 = 0325