iFixIt has posted an iPhone 4 repair manual which instructs you on replacing 10 different components including the front panel, rear panel, and battery.
Installing iPhone 4 Battery
Installing iPhone 4 Home Button
Installing iPhone 4 Speaker Enclosure
Installing iPhone 4 Dock Connector
Installing iPhone 4 Logic Board
Installing iPhone 4 Vibrator
Installing iPhone 4 Front Facing Camera
Installing iPhone 4 Rear Camera
Installing iPhone 4 Front Panel Assembly
Installing iPhone 4 Rear Panel
