I'm going to assume that UE4 is intended for DirectX 12 and up which would be quite a leap from 11.
http://www.youtube.com/watch?v=oRp_mVi969IWait what? Quite a leap? DirectX 12? ;P?
Okay let's take it from the beginning. Direct3D 8 to 9 was quite a leap because developers could now program the Vertex and Pixel shaders. This is one of the reasons Wii didn't get nay ports more or less because the engine had to be rewritten to fit the Wii API which has the same features as Direct3D 8 more or less. Direct3D 8 is really static. Adding lamps in a scene requires you to add them via the API. With 9 u merely had to add them as a variable in the pixel shader.
Direct3D 9 to 10 was quite a leap because the GPU architecture went from being devided into vertex- and pixel shader units into a unified arcihtecture where every units could do everything. Things means you didn't have to have in mind the vertex shaders could be starving while pixel shaders could keep up.
Direct3D 10 to 11 was a minor upgrade, mostly only adding tessellation, other than that it was only API changes.
Wii U has a modern GPU with modern architecture. The 4870 is more or less a 5770 or 6750 or 7750 with the lack of a proper tessellation unit.