I've installed XCode4 and I need to build for iOS 4.2.1 is there a possibility to install the "old" firmware 4.2.1 and build against it?
As far as I understand it you can only use a 4.3 Base SDK but can target also older OSes like 4.2.1 or whatever. So no need to install old firmwares.
Actually it wouldn't make sense any other way. I have a older iPod Touch which can only be upgraded to 4.2.1, so it has to be programmed targeted for 4.2.1 using the same toolset.
来源：https://stackoverflow.com/questions/5562952/xcode-4-and-ios-4-2-1