My iPhone has a very low battery life.

Hi,
I bought an IPhone from a friend of mine, who frequently complained that the IPhone had a very low battery life. I bought it from him because I wanted an IPhone so badly, that it would be okay for me to spend money on fixing the battery life.
I tried switching off all unnecessary applications and notifications (like GPS, etc), but still my IPhone lasts for only 4 hours of continuous usage. Added to the fact that there is no authorized Apple store in our city, which means that I cannot find professional consultation for my problem.
What should I do?
Thanks.
