Not sure on 4K. 1080P in good to high quality requires 6 to 10 Mbps in H.264, what do people actually stream at? Hard to say sometimes lower if their connection is crap or well... normal for cable lol. I wouldn't stream 4K if you are on cable and have a low upload speed. If you are on fiber and have 25+ Mbps I am sure it's easily possible. Have to leave some bandwidth for whatever you are playing.
Here is to hoping for H.265 support in the near future for streaming.