No matter how good the battery is on a device, we need more juice. The iPhone 4, like everything else, always seems to run down at the worst times. And the iPhone 4 also needs a case, so you might as well take care of both problems with one device.
The Boost Case will fit the bill since it has a battery inside. It has a black exterior with blue LEDs to show how much battery life is left in the device. The battery inside has 1700mAh of power and the case is just 0.69″ thick. The case can be charged alone or you can charge the case battery and the iPhone 4 battery at the same time, which is nice.