Step 1: Divide the number into separate digits:
7394 => 7 3 9 4
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
7 to 3 = 4
3 to 9 = 6
9 to 4 = 5
4 to 7 = 3
Step 3: Combine the filters to a string:
4 6 5 3 = 4653