Step 1: Divide the number into separate digits:
9340 => 9 3 4 0
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
9 to 3 = 6
3 to 4 = 1
4 to 0 = 4
0 to 9 = 9
Step 3: Combine the filters to a string:
6 1 4 9 = 6149