The battery in the iPhone 4s was giving up the ghost after a year and a half of use.
The repair went quite easy. Compared to the battery swap in my iPhone 3S, this was a breeze. I did order the Pro Tech Tool Kit, so that did make the swap easier.
The Pentalobe screws were removed easily with the Pro Tech Toolkit.
Total time to do the installation was under ten minutes.
I never had luck with the plastic smudges. The Pro Tech Toolkit comes with metal smudge tools that won't break the second you try to pry the battery out of the phone.
Be careful with the tiny screws. You do not need a lot of torque to hold them in. If you try to apply and torque at all, you will strip the threads. A thumb and your pointer finger are all that should be used on the driver to tighten.
Try to avoid touching any of the internal circuitry with your bare hands. You risk electrostatic shock to the components and the transfer of oils from your fingers may cause issues with the phone.
Take a look at the ground wire present under the screw that retains the battery's connector plug. Keep it's orientation in mind when it comes time to reassemble. Take a picture if need be.