Step 1: Divide the number into separate digits:
9049 => 9 0 4 9
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
9 to 0 = 9
0 to 4 = 4
4 to 9 = 5
9 to 9 = 0
Step 3: Combine the filters to a string:
9 4 5 0 = 9450