Before you begin, discharge your iPhone battery below 25%. A charged lithium-ion battery can catch fire and/or explode if accidentally punctured.
Power off your iPhone before beginning disassembly.
Your iPhone 4 rear cover may have either two #000 Phillips screws or Apple's 5-Point "Pentalobe" screws (second image). Check which screws you have, and ensure you also have the correct screwdriver in order to remove them.
Remove the two 3.6 mm Pentalobe or Phillips #000 screws next to the dock connector.
Be sure the driver is well seated when removing Pentalobe screws, they are very easy to strip.
During reassembly, we recommend you replace the 5-point screws with equivalent Phillips screws. Our Liberation Kit provides the tools and screws needed to replace the Pentalobe screws with Phillips screws