I did this in UE2 by using the stencil buffer to draw the portal over the main scene, and I think that is the standard way of doing this no matter what the engine is - given how heavily the stencil buffer is used by the engine though, in both UE3 and UE4, doing this would probably require expert knowledge of the internal rendering system, and a lot of customization, in order to make it work.
Disclaimer: I know next to nothing about working with the rendering system 
though this would make for a lengthy/fun and highly educational project.