Going by the name of the AP1201 Rechargeable iPhone 4 Case with Built-in Battery, it will provide some much needed juice for those power heavy owners out there. Energizer touts that the it'll be able to provide approximately twice the amount of power while still protecting the smartphone from an array of things – like normal wear and tear or even drops. You can instantly see how much juice is left with the battery case because there are LEDs hidden on the back of the case for easy viewing. You can charge the battery case via the included USB charging cable – which will also charge the iPhone at the same time.
If this is a solution that fits your needs, just expect to shell out $69.99 for it – and it's currently available through a handful of retailers.
Apple iPhone 4 Specifications | Review