Step 1: Divide the number into separate digits:
3039 => 3 0 3 9
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
3 to 0 = 3
0 to 3 = 3
3 to 9 = 6
9 to 3 = 6
Step 3: Combine the filters to a string:
3 3 6 6 = 3366