Step 1: Divide the number into separate digits:
8904 => 8 9 0 4
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
8 to 9 = 1
9 to 0 = 9
0 to 4 = 4
4 to 8 = 4
Step 3: Combine the filters to a string:
1 9 4 4 = 1944