IPhone 4S battery dies quickly?

So I've noticed my battery on my phone has been dying quickly. I turned the brightness down, made sure only one app was running, and it's still dying.
I've been in the car using 4G network and we've been driving for about 20 minutes. My phone was at 100% when we left, and now it's at 86%.
Any ideas?

I had the same problems, then I got to know that the simplest of things can kill the battery like: having your bluetooth or wifi on when you are not using it, having multitasking turned on or even use it while it is charging, when you do this it deteriorates your battery capacity as the more you use it when it is plugged in, the more depended it becomes on the electricity hence reducing your battery percentage