I am trying to find the "proper" (or perhaps, "best" would be more correct) way to write the plural of a capital letter. Specifically, when describing grades, if a person receives 4 marks of "A" then would it be written as 4 As or 4 A's?

I've seen it both ways and can find support for both - I was just curious.

I'd prefer the way with an apostrophe.
A's, B's, C's,...

I think this is a better way to avoid misunderstandings (esp. As, Es, Is, Os and Us could be regarded as special words of their own).
Quite interesting that also German does it the same way although the plural -s is very rare in the German language (usually only the words that were taken from English, abbrevations and short forms take the plural -s) and the apostrophe is actually generally wrong.
Wrong, no apostrophy!