I have looked at this question, and apparently my iMac does support mixing memory sizes. However, when I installed 2 4GB memory modules and 2 2GB memory modules, when I clicked → About This Mac → More Info... → Memory, it said that I had only installed 4 2GB modules. I have checked the packaging, and it still verifies that they are both 4GB modules. For further information, I am running Version 10.9.1 Mavericks.
If you use only the 4GB modules, what does it say then? If it still see's them as 2GB each, it might be the RAM - who knows, maybe they just put the wrong sticker on it ;)