Step 1: Divide the number into separate digits:
3499 => 3 4 9 9
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
3 to 4 = 1
4 to 9 = 5
9 to 9 = 0
9 to 3 = 6
Step 3: Combine the filters to a string:
1 5 0 6 = 1506