I just use a Bookback for iPhone 4 by Dodocase and a screen protector I purchased on Amazon for 6 for $4. The Bookback is just a piece of leather and doesn't offer much in the way of drop protection but it has an awesome feel and keeps the profile of the phone much smaller. I tried a regular graphite case when they were giving them away and it was good but I found it to be bulky.
http://www.dodocase.com/products/bookback-for-iphone4