My wife's iPhone 4S was not holding a charge very long. After being on the charger over night and on stand by the battery would loose more than 50% to 60% later the next day. Two days later I ordered a new battery from IFIXIT. The day after the order was placed the phone went completely dead.
Replacing 4S batteries are very easy to do. A few days later I put the new battery in. Yes, I replaced two batteries with-in a week.
I followed IFIXIT's instructions and watched each step by step carefully.
It is a simple phone to replace the battery and took me about 10 to 15 minutes. Removing the battery from the phone went well. Laying the new battery in and using a small tweezer to hold the ground in place. After it's put back together I powered it on and works like new again.
Watch IFIXIT's instructions until you fully understand them.
Use the tools that come with the kit and a small pointed tweezer.
A magnifier is a big help for the small screws.
Keep track of the screws and small parts in a container
Take your time, don't rush. It's very simple to do.