Step 1: Divide the number into separate digits:
6240 => 6 2 4 0
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
6 to 2 = 4
2 to 4 = 2
4 to 0 = 4
0 to 6 = 6
Step 3: Combine the filters to a string:
4 2 4 6 = 4246