Step 1: Divide the number into separate digits:
5402 => 5 4 0 2
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
5 to 4 = 1
4 to 0 = 4
0 to 2 = 2
2 to 5 = 3
Step 3: Combine the filters to a string:
1 4 2 3 = 1423