Unreal Engine 4 is up and running on next-gen systems, according to Epic Games vice president Mark Rein. Sadly he can't confirm what those systems are.
Speaking at the DICE summit, Rein said that the upcoming next-gen game engine is running on a variety of hardware, "including systems we can’t name yet". He also told G4TV that the new engine will be revealed this year.
"People are going to be shocked later this year when they see Unreal Engine 4 and how much more profound an effect it will have," he said.
Presumably the unnameable systems are the next Xbox and PlayStation, although it's not out of the question that Epic has got the engine running on the Wii U - but it's unlikely.