Step 1: Divide the number into separate digits:
7340 => 7 3 4 0
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
7 to 3 = 4
3 to 4 = 1
4 to 0 = 4
0 to 7 = 7
Step 3: Combine the filters to a string:
4 1 4 7 = 4147