I don't see a reason porting UE4 to Wii U if UE3 will do the same things. Porting UE4 to Wii U would mean that Wii U would have fewer resources than using UE3. There is always bottom (minimum spec) line that needs to be first taken care off. But yeah UE4 can be on WiiU but there is simply no point in it if there is UE3.5
Well I don't really want to go too far into it since this is the wrong thread. But what I will say is a big reason for having a certain engine on a system is the ability to use the same tools on numerous platforms, making the development process easier and cheaper. So if you're porting a few UE4 based games to WiiU it could easily be worth porting the engine first to make the process easier.
Also while UE4 should require more computational resources and RAM during the development process (due to new tools/dev features). I don't see why the final result would require more resources for the same level of game as UE3. Or has there been info suggesting UE4 is less efficient than UE3 that I missed?