Luckily, replacing the battery on the iPhone 4 requires minimal disassembly. If your battery is swollen, [[What to do with a swollen battery|take appropriate precautions|new_window=true]].
For optimal performance, after completing this guide, calibrate your newly installed battery: Charge it to 100% and keep charging it for at least two more hours. Then use your iPhone until it shuts off due to low battery. Finally, charge it uninterrupted to 100%.