EDIT: Higher quality version available for download in here (might need to log in): https://vimeo.com/121146483
...or in here: http://www.gamersyde.com/news_unreal_engine_open_world_video-16335_en.html
Epic games said:
Created by Epic Games, Kite is a short film about a boy as he pursues his stray kite through a diverse and beautifully realized landscape. Everything you see is generated in real time by Unreal Engine 4 at 30fps. This demo showcases Unreal Engine large world features, photo reconstruction and procedural asset placement. Get Unreal and get going for free at www.unrealengine.com.
Here's the presentation/making-of/behind the scenes and Q&A: http://www.twitch.tv/unrealengine/b/632598238?t=45m28s
The GDC presentation is now on YouTube: GDC 2015: Creating the Open World Kite Real-Time Demo in Unreal Engine 4