Apple doesn't want you to know how to do this - at least not by yourself - because they can't earn any moolah off of it. But with just a few basic tools you probably already have lying around the house you can remove the back cover of your iPhone 4 easily and then replace the cover with another one of your choosing.
Think of this project as an easy way of customizing an otherwise very uniform iPhone that looks like everyone else's. You can also watch this iPhone tutorial if you shattered the back glass and would like to replace it. Works on at&t and verizon.