Step 1: Divide the number into separate digits:
8184 => 8 1 8 4
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
8 to 1 = 7
1 to 8 = 7
8 to 4 = 4
4 to 8 = 4
Step 3: Combine the filters to a string:
7 7 4 4 = 7744