Having previously covered all of MusicSkins' iPod, iPhone, and iPad stickers, there isn't much left to surprise us in the latest version -- Skins for iPhone 4 ($15) -- which covers only the new iPhone's back surface without attempting to stretch to its sides or face. That's different from the iPhone 3G/3GS version alongside it, which wrapped around the prior model's plastic body, stopping short of the bezel, and the iPad version, which also included…