Step 1: Divide the number into separate digits:
5904 => 5 9 0 4
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
5 to 9 = 4
9 to 0 = 9
0 to 4 = 4
4 to 5 = 1
Step 3: Combine the filters to a string:
4 9 4 1 = 4941