If you're planning to take an overnight hike with your iPhone along for navigation, you'll probably need an external battery to keep the phone juiced up. The Incipio OffGrid is a case plus battery combo that doubles the battery life of the iPhone. The case adds 1.9 ounces, not much more than a standard case weighs. Cost is about $40 online, again, not much more than you'd pay for a quality case that doesn't include a battery.
Here's the web site http://www.incipio.com/iphone-4-battery-case/
and here's an internet vendor with a low price http://www.amazon.com/Incipio-iPhone-offGRID-Backup-Battery/dp/B004UDRREK
Here's the web site http://www.incipio.com/iphone-4-battery-case/
and here's an internet vendor with a low price http://www.amazon.com/Incipio-iPhone-offGRID-Backup-Battery/dp/B004UDRREK