When it comes to iPhone batteries, iPhone 4 users is advice to consult service providers such as Apple (QCD) or Apple/Maxis/Celcom/Digi if faced with battery problems. They will most likely replace your old with a new iPhone 4. Please understand that refurbished iPhones are not that scary as they are made up to be. They still have to go through the mandatory quality control check for every new iPhone that goes out of the factory!
Beware of fake batteries easily available everywhere on the web. Your iPhone will be reporting wrong battery status, sudden shut downs and when restoring your iPhone you get an error 29 message.
On another note, please be advice NOT let your battery go to zero. As these are Li-ion Polymer batteries, please ensure that your iPhone is charge with a minimum 20% battery life.
*Tips: Once a month, allow the battery to drain below 20%, charge it up fully to 100% and leave it for 2 hours. This trickle charge will ensure it reaches full charge status