Create a basic app that creates a single video from four videos

The app will be very simple, it does not need a UI outside of 5 buttons that can be arranged in any fashion as it is just a prototype. Four buttons, tapping each of them allows you to select a video from the camera roll. One save button. Tapping the save button will render all of the videos into a single video in a grid as shown in the example image. The videos will all play at the same time. The length of the final rendered video should be as long as the longest selected video, the shorter length videos will loop until the video ends. The final xcode project should use the built in AVFoundation library etc rather than third party libraries that could cause it to run slowly.