The iPhone 4's glass backing is remarkably fragile—even more so than the other iPhone models. If you'd like to keep it protected without making the phone bulky with a case, consider replacing the back with this metal back cover instead.
For a mere $US12, you can save your iPhone from accidents by replacing the back cover with this nice, beveled metal one. There are, of course, others floating around the net, but tech blog Unplgged recommends this one for its low price, beveled back, and super easy installation—all it takes is a bit of unscrewing (and it looks pretty darn good too). I still wouldn't be too casual about dropping your iPhone, but if you're prone to accidents and don't like big cases, this is another great way to go.