Though, my own advice here, popping up a dialog box that says, "If you had a newer phone, you'd be able to do this!" is really just rubbing your user's face in the fact that they've got an older phone, now isn't it? A better way to handle it would be to 1) make it clear in your AppStore page that certain features won't be available on non iPhone 4 phones and 2) make your app's behavior degrade gracefully on non iPhone 4s.