For a while i have bought this great Case on ebay for my S4.
It gives you're S4 a whole new look and also a good protection!
(The backcover is made of Gorilla-glass)
Installation is quit easy, just take your'e old batterycover of and put the phone in the case, than put the new backcover on it!
for german users you can buy it here:
for the others just search for Luphi Case, Alu Bumper, CNC Alu Case or similar. Maybe you have to buy it from a seller on ebay china.