According to Apple, the iPhone 4 and 4S batteries are good for around 400 full charge cycles (from 0% to 100%). This means your phone could easily outlast its battery. However, before you walk into your nearest Apple store and hand over £60 to get a replacement installed, why not do it yourself?
Before you start, remember there's a strong possibility you'll invalidate your iPhone warranty by doing this. If you have any doubts about it, please use the Apple store option.
The iPhone may look impenetrable, but there are only three screws (four with the 4S) that need to be removed in order to get the battery out, and anyone that can wield a screwdriver should be able to complete the job in about five minutes.
Genuine Apple replacement batteries are freely available via eBay for roughly £10, but make sure you order the correct model fort your phone. The iPhone 4's battery has a part number of 616-0520 or 616-0513, while the 4S requires a 616-0580.
The only other thing you'll need is a screwdriver. Some iPhone 4 models have standard Philips 00 screws, so a Philips 00 screwdriver is all you'll need. If you find your iPhone has five-pointed star screws, you'll need to buy a 'Pentalobe' screwdriver - again, these are cheapest from eBay - to remove them.
1. Turn your phone off by holding the power button for a few seconds until the red slider appears. Slide it to power the phone off.
2. Use your Philips 00 or Pentalobe screwdriver to remove the two screws on the bottom edge of your iPhone. Keep them somewhere safe, as they're extremely easy to lose.
3. Slide the rear cover upwards by a few millimetres and lift it off. If it's stuck press the palm of your hand against the entire back panel to get the maximum friction.
4. Remove the single Philips screw which secures the battery connector to the motherboard. If you have a 4S (shown below), there are two screws which need to be removed.
5. Use a fingernail (or a special plastic pry tool if you prefer) to carefully lift the connector up from the top and bottom edges. Note that on the iPhone 4 there's a black contact clip below the connector which the screw threads through. You can carefully remove this or leave it in place. Take note of how it fits, so you can replace it if it falls out.
6. Use the clear plastic tab to pull the battery up and out of the phone. The battery is glued into place, and may require considerable force on the tab to lift it. Pull the tab with progressively more force, and use a pry tool if necessary to help release it.
7. Before fitting the new battery, make sure the contact clip is properly positioned. The iPhone 4 clip is shown below, and fits in this orientation. Press the battery connector into place, and tighten the screw to secure it.
8. Replace the rear cover and two screws to secure it in place. Connect your mains charger to initially charge the new battery.