Step 1: Divide the number into separate digits:
8604 => 8 6 0 4
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
8 to 6 = 2
6 to 0 = 6
0 to 4 = 4
4 to 8 = 4
Step 3: Combine the filters to a string:
2 6 4 4 = 2644