My app uses Storyboard and on the storyboard it looks fine with the bigger 4" screen. But, on the simulator it opens the 3.5" app and puts some black space up and down the screen.
Storyboard:
Simulator:
You need ensure that you have splash screen asset that is sized for the 4" screen. This is normally called something like Default-568h@2x.png
and needs to be added to the project settings.