Step 1: Divide the number into separate digits:
1604 => 1 6 0 4
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
1 to 6 = 5
6 to 0 = 6
0 to 4 = 4
4 to 1 = 3
Step 3: Combine the filters to a string:
5 6 4 3 = 5643