how do you see how your function performed in each test case?

It is a question not an issue.

I'm failing test 4. Does anyone have any ideas?

Test4
Test Failed
Expected is <System.String[5]>, actual is <System.String[6]>
Values differ at index [1]
Expected string length 3 but was 2. Strings differ at index 0.
Expected: "cod"
But was: "ar"
-----------^