Step 1: Divide the number into separate digits:
7904 => 7 9 0 4
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
7 to 9 = 2
9 to 0 = 9
0 to 4 = 4
4 to 7 = 3
Step 3: Combine the filters to a string:
2 9 4 3 = 2943