Back panel on your iPhone 4 has either two #000 Phillips screws or Apple's 5-Point "Pentalobe" screws. Make sure to check which screws are used; also make sure that you have the appropriate screwdriver in order to remove them.
Remove the two 3.6 mm Pentalobe or Phillips #000 screws next to the dock connector.
While reassembling the phone, we suggest you to replace the 5-point screws with equivalent Phillips screws. We have all the necessary tools and screws needed to replace the Pentalobe screws with Phillips screws.