The iPhone 4s records 1080p video which is a 16:9 widescreen ratio. The screen on the iPhone 4s is a 3:2 ratio. The video is going to be letterboxed when watched at full resolution. There is a button in the top right hand corner of the video with 2 arrows pointed diagonally that will expand the video to play full screen. If this is not what your wanting but to actually record full screen you can always use an app like Filmic to change the recording resolution to something like 640x480. Other than these two options I'm not sure you can do what your wanting to do.