Set iOS 6 as the Base SDK and use the Auto Layout feature to make screens that can scale for all type of screens. You'll need Xcode 4.5 to do this.
Get started with Auto Layout here:
http://www.raywenderlich.com/20881/beginning-auto-layout-part-1-of-2
http://www.raywenderlich.com/20897/beginning-auto-layout-part-2-of-2
If you still want to support iOS 4.0, have separate .xib files for different screen sizes and load them appropriately at launch.
To load different nib files based on your screen size, in your app delegate, you will need to add/replace the following code in - (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
CGRect screenBounds = [[UIScreen mainScreen] bounds];
if (screenBounds.size.height == 568) {
self.viewController = [[ViewController alloc] initWithNibName:@"ViewController_4inch" bundle:nil];
} else {
self.viewController = [[ViewController alloc] initWithNibName:@"ViewController" bundle:nil];
}
where ViewController_4inch
is the name of the nib file that is designed for iPhone 5 screen