So after playing in low light tonight someone sent me to this thread. I had a pretty bad test shoot with 4k60p late afternoon, dusky. Mushy everything and the noise was not like grain - it's horrible. I did render out a short clip and this is WITH neat video noise reduction on it.
I seem to remember a while ago reading about this similar issues with GoPro Hero 4 cameras - the setting I seem to recall was to use 2.7k which would give a better image than the 4k setting. Also I think it pulls into an HD 1920 frame better than the 4k.
here's the clip from tonight...