battery replacement for 4s
Does Apple replace the iPhone 4s battery?
iPhone 4s, iOS 9
Question marked as
Apple can generally replace an iPhone 4s battery if the screen hasn't been damaged and it doesn't have past liquid damage. However it is limited to what Apple has in stock as they aren't making the batteries anymore.
Also, I believe it is $79 which is really more than an iPhone 4s is worth these days so keep that in mind. You would be better off getting a used replacement honestly.