You need to include the 4" Launch Image to utilize the entire 4" screen.
From iPhone Development 101:
Note that the Default-568h@2x.png image is required for an app to display in full-screen mode on the iPhone 5.
You need to include these anyway (Apple will auto-reject your app without a launch image for both the 3.5" retina and 4" retina displays). Just add these images, named exactly what they are on that page to your src
directory in Flash Builder and you should be good to go.