Step 1: Divide the number into separate digits:
9400 => 9 4 0 0
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
9 to 4 = 5
4 to 0 = 4
0 to 0 = 0
0 to 9 = 9
Step 3: Combine the filters to a string:
5 4 0 9 = 5409