Battery percentage is inaccurate!

Everytime my iPhone 4's battery gets to about the halfway point (with or without the battery percentage on) it dies! It's extremely frustrating thinking you have all this battery left and then having your phone shut off on you. I've reset, restored, and set up as a new phone on iTunes. Any suggestions?
I'm not sure if it's a big enough problem that apple would replace my phone? If they even have any in stock..