GPU rendering is possible but usability is split into two camps of CUDA and OpenCL so has the same problem in Unreal 4 as hardware physics where nVida supports CUDA and ATI supports OpenCL.
Example of CUDA based rendering.
As mentioned video memory would become a problem for any kind of real time rendering but environment lightmass rendering could be just as fast, if not faster, then network based rendering as well as a huge bump in per frame rendering of 4k images.
At the very least would make for a very useful 3rd party product as a rending solution for Unreal 4 like a V-ray or Renderman plug in solution for say 3ds Max. 