MiLi offers the Power Spring 4 (model HI-C23) that claimed to be the thinnest external battery for iPhone 4. MiLi’s iPhone 4 battery case boasts 1600mAh capacity that provides up to 450 hours of additional standby time, 7.5 hours of talk time, 7.5 hours of 3G internet use, 13.3 hours of WiFi internet, 15 hours of video playback and 45 hours of audio playback.
The MiLi Power Spring 4 measures 124 x 62 x 15mm and is available in various colors. It uses mini USB port for charging. It is available for $$84.58.