4. Incipio OffGRID Pro
Incipio's OffGRID Pro provides extra battery life for those who need a bit of extra juice when they're on the go. The OffGRID Pro is admittedly heavy (as you'd expect) but the fit is snug and sturdy, and the extra battery only adds a little bulk to the back of your precious iPhone.
The case still gives you full access to all your buttons, ports and camera and if anything, the curved back makes the phone more comfortable to hold. This is hands-down the best battery pack available for the iPhone 4S.
Price: $100 (Available April 2012) | Buy: Incipio OffGRID Pro for iPhone 4s