iPhone 4 has a swappable back plate? YES. And changing it out is super easy to do, as Adriana finds out. Here, the iPhone 4′s glass backplate gets taken off and replaced with a metal one from iPhone components site http://cnn.cn. Check it out.
[NOTE: Opening your iPhone CAN void its warranty. If you attempt this, act at your own risk. And for goodness' sake, definitely hang onto the original backing. You never know when you need to pop that back on, so you can take it to the store.]