When developing Windows Phone 7 apps, one is inclined toward the navigation model for moving between pages. But it is worth calling out that there is a different way to proceed, which is to simply load your new page in the root visual, as follows:
App.Current.RootVisual = new Screen4();
This has the advantage and disadvantage of entirely bypassing the navigation framework, which means that the back button always exits your app. I say this is an advantage because for certain applications, that's the very behavior you seek, and you can find yourself getting into a lot of trouble with the navigtation history problems, which can cause your app to end up in a loop and not be able to be exited (since there is no App.Exit() method for Windows Phone 7 - see this blog post for the reasoning). I say disadvantage because you lose the ability to do navigation transitions, as explained in this post.
Nonetheless, it is worth considering up front when you start an application as to whether you should use the NavigationService model or the RootVisual model.