Step 1: Divide the number into separate digits:
7489 => 7 4 8 9
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
7 to 4 = 3
4 to 8 = 4
8 to 9 = 1
9 to 7 = 2
Step 3: Combine the filters to a string:
3 4 1 2 = 3412