In UE3 you would use a 2D capture actor to make accurate dynamic reflections for floors and mirrors, that’s not available yet in UE4 there’s just a capture actor that acts like you would use a security footage type thing, some people have used that to get close to an accurate mirror type effect, but it’s not quite there.
Reflections are always a complicated issue, because it’s currently not possible to do them accurately in real-time, so engines use tricks, like prerendered reflection probes which aren’t accurate but look good, or screen space effects which are fairly accurate but don’t work with surfaces that aren’t rendered, or the capture actors which only work on flat surfaces.