Step 1: Divide the number into separate digits:
4035 => 4 0 3 5
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
4 to 0 = 4
0 to 3 = 3
3 to 5 = 2
5 to 4 = 1
Step 3: Combine the filters to a string:
4 3 2 1 = 4321