FYI, this is iPhone 4 only and it works like a charm! I haven't seen anyone really posting this and if it's been posted, my bad. This is probably for people who have legitimate carrier iPhones and just want to preserve the baseband for future unlocking purposes instead of those people who absolutely rely on the unlock for the iPhone 4s to work. http://www.iphone4jailbreak.org/how-to-upgrade-ios-4-2-1-preserving-baseband.html The instructions and the video I feel is pretty self explanatory! I've attached a screenshot, I'm at 4.2.1 and my baseband is still unlockable at 01.59.00.