在您的应用程序委托中，您需要在 - (BOOL)application：（UIApplication *）application didFinishLaunchingWithOptions：（NSDictionary *）launchOptions 中添加/替换以下代码

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];
}

其中ViewController_4inch是为iPhone 5屏幕设计的nib文件的名称

更新（特定于Storyboard的答案）：

要在启动时加载不同的故事板，请使用此代码：

    CGSize iOSDeviceScreenSize = [[UIScreen mainScreen] bounds].size;
    if (iOSDeviceScreenSize.height == 480)
    {   
        // Instantiate a new storyboard object using the storyboard file named Storyboard_iPhone35
        UIStoryboard *iPhone35Storyboard = [UIStoryboard storyboardWithName:@"Storyboard_iPhone35" bundle:nil];
        // Instantiate the initial view controller object from the storyboard
        UIViewController *initialViewController = [iPhone35Storyboard instantiateInitialViewController];
        // Instantiate a UIWindow object and initialize it with the screen size of the iOS device
        self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
        // Set the initial view controller to be the root view controller of the window object
        self.window.rootViewController  = initialViewController;
        // Set the window object to be the key window and show it
        [self.window makeKeyAndVisible];
    }
    if (iOSDeviceScreenSize.height == 568)
    {   // iPhone 5 and iPod Touch 5th generation: 4 inch screen
        // Instantiate a new storyboard object using the storyboard file named Storyboard_iPhone4
        UIStoryboard *iPhone4Storyboard = [UIStoryboard storyboardWithName:@"Storyboard_iPhone4" bundle:nil];
        UIViewController *initialViewController = [iPhone4Storyboard instantiateInitialViewController];
        self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
        self.window.rootViewController  = initialViewController;
        [self.window makeKeyAndVisible];
    }