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 Default@2x.png for iPhone4 and Default-568@2x.png 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 Default-568@2x.png 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.