Step 1: Divide the number into separate digits:
8042 => 8 0 4 2
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
8 to 0 = 8
0 to 4 = 4
4 to 2 = 2
2 to 8 = 6
Step 3: Combine the filters to a string:
8 4 2 6 = 8426