Is Iphone 4 and Iphone 4GS have same kind of battery?

Hello friends I want to replace old battery of my friend's I phone 4 GS, also she have I phone 4. Could you please tell me are these both phone have same battery according to their specification or what. Actually my friend is thinking to buy battery online so we want to make sure which battery should be use in both phone.

There's no iphone 4gs