Step 1: Divide the number into separate digits:
1042 => 1 0 4 2
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
1 to 0 = 1
0 to 4 = 4
4 to 2 = 2
2 to 1 = 1
Step 3: Combine the filters to a string:
1 4 2 1 = 1421