5 - 30 minutos
The iPhone 4 was being used nearly 24/7 by either my wife or our young daughter. After this type of abuse and heavy downloading on the 3G network over a two year span the battery decided it couldn't do it anymore and would only hold a 100% charge less than six hours before being depleted to zero.
I'm not a religious man but I found ifixit.com on God's will alone. Or because it was the first site at the top of the google search "how to replace iPhone 4 battery".
After reading over how to change the battery I decided I can do it, plus it is much cheaper to do than buying the iPhone 5.
I ordered the battery, liberation kit and the small, blue pry things. To my surprise they arrived from the left coast in only a few days. Now the pressure was on. I got one of the pentelobe screws removed but the other would not budge. After closer inspection of the screw and the pentelobe driver that came with the liberation kit I determined that the driver was a POS.
I called ifixit.com right away and spoke with a guy named Scott. I told him my situation and asked if they had a better driver I could buy. He said they do sell better ones but he'd put one of their own brand in the mail, no charge. This confirms customer service is not dead in America and that guys named Scott are awesome.
Three days later I had the much improved ifixit pentalobe driver and in less than five minutes I had the old battery out and the new one in, case screwed back on and phone on charge.
Read the demonstration several times over before attempting and when you take the cover off pay close attention to how things are supposed to look befor you go unscrewing stuff.