As far as I know, the UDK is a free version of UE3 for public non commercial usage back in 2009. That demo also looks similar to one I've seen a couple years back. I don't think it's quite related to UE4. I don't think anyone's even made a tech demo showing foliage and waterfalls with it that i'm aware of. So how well it compares I guess we'll have to wait. Either way, I still think a Stalker game would look awesome with it or Cryengine 3, Dunia, Frostbite 3 or Chrome engine 5 for that matter.