I believe they used the stencil buffer in UDK to render (from a separate camera) to a part of the screen covered by a portal polygon, same technique that was used by Valve. UE4 uses a deferred renderer, whereas UDK used a forward renderer. I’m not sure how this complicates things, but that might be the reason why it was not included.
Be that as it may, I too would like to see this awesome feature in UE4, so here’s my vote 