Step 1: Divide the number into separate digits:
5900 => 5 9 0 0
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 0 = 0
0 to 5 = 5
Step 3: Combine the filters to a string:
4 9 0 5 = 4905