As of June, 2012 I have owned an iPhone 4 for 18 months. In the past month the battery life has dramatically shortened. A full charge in the morning is down to 40% or less by 5 PM. I tried all of the normal tricks to improve battery life but it was obvious that the battery itself was worn out.
A quick search on EBay turned up many vendors selling very cheap batteries, so it was worth a shot. I got lucky and the seller sent a small toolkit with the battery which was handy considering the tiny screws on the bottom of the iPhone 4 are 00 Phillips if you are lucky or pentalobe screws if you’re not. Either way, the tool kit works with either type.
After swapping the battery and charging it overnight, it now drains to just 89-90% by 5 PM! Very worthy investment!
UPDATE: 5-Sept, 2012 – The replacement battery is discharging to about 40% by the end of the day again. I suspect that the replacement isn’t as good as the actual Apple battery. I will order another and try it.