Enter the $50 PowerPak XT from Technocel which won an Innovations award at the 2011 CES. Instead of carrying an A/C adapter and a spare battery, the PowerPak is all of the above. The unit is small and lightweight and pretty much foolproof. It has a standard USB port plus A/C prongs that fold into the unit. They have offered PowerPak models before, but this newer PowerPak XT's larger 1600mAh lithium battery adds up to 4.5 hours of talk time. That's not enough to charge an iPad but a good source of energy for an iPhone. It comes with plugs for just about every device you can think of, except unfortunately Apple's 30 pin dock connector, so you will have to supply that. The device's sole button displays how much power is left, via four LEDs. Carrying the PowerPak and a small dock cable gives you battery power plus the ability to recharge both the unit and your iPhone when you find an A/C outlet.
Mophie offers another way to stay charged for an extended amount of time. Their juice pack air delivers 1500 mAh and the juice pack plus offers a whopping 2000 mAh of power! The mophie cases do add noticeable thickness to the otherwise svelte iPhone 4, but the resulting device is just a bit bigger than an original iPhone in a standard case.
The supplied microUSB cable can be used to sync your iPhone and charge both the case and iPhone. If you are going to keep your iPhone 4 in a case anyway, and you worry about running out of power, consider the mophie offerings, now available in several new colors, introduced at CES, that complement the iPhone 4.