Step 1: Divide the number into separate digits:
9210 => 9 2 1 0
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
9 to 2 = 7
2 to 1 = 1
1 to 0 = 1
0 to 9 = 9
Step 3: Combine the filters to a string:
7 1 1 9 = 7119