How do I control the relative position of views, especially I wish my app to run on 3.5 inch display and 4 inch display seamlessly?
Check out this: iOS 6 apps - how to deal with iPhone 5 screen size? and How to add iPhone 5 large screen support to iOS apps in Xcode?
But it's done for you for the most part with auto layout. Click on your project in Xcode and go to the Summary tab to add the different screen size launch screen for your app.