You can install any amount of RAM up to 16GB, as long as each module is 4GB max. So, if you want 8GB, the best combination is 4+4GB. If you simply put one stick of 8GB inside, it will either not be recognized at all, or recognized as 4GB stick.
Double check that you actually have all the RAM available; it might have detected it as 8GB, but only leave 4GB as usable. You can see that in system properties in Windows (the section that mentions memory).