You can actually get an iPhone 4S replacement battery + tools to change the battery for less then $10 dollars. Certainly a-lot cheaper than replacing the whole phone.
Here is one for $9 http://www.ebay.com/itm/1430mAh-Internal-Replacement-3-7V-Li...
Here is one for $9 http://www.ebay.com/itm/1430mAh-Internal-Replacement-3-7V-Li...
Also updated it to iOS 8 GM and boy was that a mistake. Everything is slow, lot's of bugs.