This type of scene is absolutely possible within UE4, especially after the 4.7 update. The process would go something like this.
- Get Hight map out of something like world machine and import to Landscape
- Craft up custom landscape material to give a base ground
- Load in all sorts of Speed Tree vegetation into the landscape vegetation painter tool
- Paint the Landscape with your trees, shrubs, etc.
- Add in some Meshes for houses, etc.
- Add in the FPS content
- Press Play and your off to the races!
That would be a good staring point to make a game. You would then need to add in all of your game play logic with either C++ or Blueprint.