Step 1: Divide the number into separate digits:
0438 => 0 4 3 8
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
0 to 4 = 4
4 to 3 = 1
3 to 8 = 5
8 to 0 = 8
Step 3: Combine the filters to a string:
4 1 5 8 = 4158