The video output resolution of the set-top box would be set to 2160p. This does not mean that all the channels would be in 4K resolution. Currently the only content available in 4K is the Olympics content and Netflix with a valid subscription. More informatio on the video output display could be found by visiting HERE.
Just hooked up my new Sony wide screen to x-finity x1 box and all channels are showing as 4k 3840x2160,is that correct??
You only mentioned the x1 cable-box. Do you also have the 4k DVR? If not, you are only seeing an upscaled version to 4K. The actual cable channels would only be: (1920 x 1080) / 60i, (1280 x 720) / 60p, or a SD (480p) channel.