Step 1: Divide the number into separate digits:
6216 => 6 2 1 6
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
6 to 2 = 4
2 to 1 = 1
1 to 6 = 5
6 to 6 = 0
Step 3: Combine the filters to a string:
4 1 5 0 = 4150