The Galaxy S4 and Note 3 are made from high quality polycarbonate plastic. Yes it comes in quality grades and no it's not cheap. That's a myth propagated by Apple and the pro-Apple press.
In fact the hypocrisy is that no reviewer has said Apple is using cheap plastic on the iPhone 5C, and it's made from the same material as these two Galaxies. I know plastics very well, sadly Apples so influential they have everyone convinced Aluminum is the holy grail. When in reality it dents, scratches, and transfers 100% of the shock of impact upon dropping which assures a broken display.