When Apple introduced the iPhone 4 on 24 June 2010 it also changed the battery that was in previous iPhone models. The battery in the iPhone 4 either has 16GB or 32 GB capacity. The iPhone 4 battery replacement unit is a 3.7V 1420 mAh Li-Polymer cell battery. Also the iPhone 4 battery replacement is a bit easier than replacing the batteries on the older versions 3G or 3GS and certainly easier than the iPhone 2G.
If you are at ease with working on rather small electronic units then you can order an iPhone 4 battery replacement kit. This replacement kit will include the battery, a plastic case opening tool to remove the rear panel which is sometimes called an iPod opening tool, a Phillips head screwdriver size #00 or as some of the iPhone 4s have an iPhone 4 5-point screw, called pentalobe screws, you will need the iPhone 4 5-point screwdriver to remove those screws. The kit should include an instruction page or give you an online website that has the replacement instructions in video or picture format.
When you go to replace the battery on the iPhone 4 you will need to remove the rear panel and also remove the battery. You begin by making sure your iPhone 4 is turned off. Then you take off the rear panel by removing the outside screws near the connector for the dock. Then you push the rear panel to remove. Next you have to take off the screw connecting the battery connector to the motherboard. You have to take the battery connector up very carefully so you dont pull or jerk it off. Also remove the antenna contact. Then you have to take out the battery using the plastic tab that is sticking up at the side of the battery. Again this has to be preformed very carefully so you dont damage anything inside the back of the iPhone.
When you insert your new battery you will have to reverse this procedure. Of course if you do it yourself you void any Apple warranty you may have.
Since Apple generally has a one year warranty if your iPhone is less than a year old and it will be if you purchased it recently then just send it back to Apple for replacement as this is the safest way to do it. But when you get the iPhone back from Apple you will not have any data in it so make sure you have backed all of your data up with an iTunes backup. Make sure you back up your iTunes, contacts, photos you want to keep, any email account settings, text messages you want to save and so forth. If you choose to send your iPhone to a third party repair place such as an internet site that specializes in iPhone repair you would get back your iPhone 4 with your data restored in it. If you go this route you will have to remember to send them any unlock passwords too.