Step 1: Divide the number into separate digits:
7704 => 7 7 0 4
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
7 to 7 = 0
7 to 0 = 7
0 to 4 = 4
4 to 7 = 3
Step 3: Combine the filters to a string:
0 7 4 3 = 0743