I do. The latest Xfinity box does 4k. I found out when my DVR died and had to go to Xfinity to replace the box.
Originally, I ran the picture to my TV via HDMI ARC (only 1080p) when watching the Roku or DVD, but broadcast sound to my 5.1 sound bar from the cable box via optical cable.
The new 4K box eliminated all the extra ports, so I only have one HDMI. Problem was that the HDMI ARC port on the TV won't pass the 4K, but I need the HDMI ARC to send sound to the sound bar. It turns out that the HDMI 3 on my TV has the 4K input, but it won't return sound to the sound bar.
I had to get an HDMI switch box with separate sound out to make it all work.
-PJ
Interesting...at least you figured it out. :)
I imagine, just like when HD first arrived that there are not too many 4k channels available at the moment..