Step 1: Divide the number into separate digits:
7439 => 7 4 3 9
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
7 to 4 = 3
4 to 3 = 1
3 to 9 = 6
9 to 7 = 2
Step 3: Combine the filters to a string:
3 1 6 2 = 3162