Step 1: Divide the number into separate digits:
4090 => 4 0 9 0
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
4 to 0 = 4
0 to 9 = 9
9 to 0 = 9
0 to 4 = 4
Step 3: Combine the filters to a string:
4 9 9 4 = 4994