2 different xib is a kind of solution but you need to play with AutoResizing property of View if you don't want to create different xib's. Also you need to use [email protected] for iPhone4 and [email protected] for iPhone5, these are the SplashScreen images to identify whther app is running on iPhone4 or iPhone4.
If you have created iPhone4 compatible application then just add [email protected] image. Your application will work for iPhone5. But you need to handle UI for this. For example if you are using UITableView so just use Flexible height instead of hardcore frame.