The original battery would no longer hold a charge. Since hunting Pokemon takes a lot of battery life, we replaced the battery for the iPhone 4s that my son is using for Pokemon Go.
Extremely easy compared to the iPhone 5. Why did Apple abandon the removable back design of the 4S?
Take this opportunity to carefully remove any debris that has gathered around the edge of the back panel that meet up with the main part of the phone to insure that the back plate seats properly during reassembly.