Step 1: Divide the number into separate digits:
0731 => 0 7 3 1
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
0 to 7 = 7
7 to 3 = 4
3 to 1 = 2
1 to 0 = 1
Step 3: Combine the filters to a string:
7 4 2 1 = 7421