During the 2015 Games Developer Conference Epic showed off to the world Unreal Engine 4's ability to render massive open worlds full of both flora and fauna. With the release of Unreal Engine 4.8 the tools used to create this demo, referred to as the Open World Tools, are now available for use in your UE4 projects.
Please note that the Procedural Foliage Tool is still in an experimental state which means that you could run into bugs or stability issues while using the tool.