I wouldn't mess with anything besides a class 10 on your GS4. Anything below c10 is slowing down your amazing phone
While on that topic, I would only put SD cards from Samsung in. I've had bad luck with generic brands (lots of data corruption, lost a lot of good pictures) and Sandisk.
Hope this helps!
PS. I'm currently using a Samsung 32gb class 10 plus SD card, and it's been flawless.