Wii U already can run something akin to Unreal Engine 3.9 but I find this whole topic much less noteworthy than many are making it out be. While details on feature requirements for the new engine are nearly zero, comments from Epic suggest that there should be no reason why the Unreal Engine 4 wont be able to scale down to work on the Wii U. Just remember that that its not power that is the main factor for an engine, rather, its the features offered by the hardware and the age of the hardware. For example, in theory, if there were to be an Unreal Engine 4 right now, it possibly could be scaled down in terms of power to work on the PS3 and 360 because they support the features that are considered standard across the industry. However, in reality, when the Unreal Engine 4 is released we probably wont see much Unreal Engine 4 on them because those systems are aging. The modern hardware going into Wii U could be new enough to support the Unreal Engine 4 at least on a scaled down level in terms of power.
See, this is specifically the issue that was encountered with the Wii, which Nintendo wants to avoid. The problem wasnt so much that the hardware was weak, rather, it was that the hardware didnt follow industry standards at the time and therefore impossible for developers to easily port the newest engines to the Wii. The Wii U solves this and more. More than a year ago, the Wii U dev kits were already souped up Xbox 360′s in some ways. So, developers were basically just dragging and dropping their PC and 360 code onto the early Wii U hardware last year, even before any new software and projects had begun development. This is how games like Darksiders 2 were up and running on Wii U hardware for E3 2011 after only a matter of weeks in development, though Nintendo finally decided not to show the instant port those developers had completed.