I don't know if we'll ever see a linear channel that broadcasts in 4K 24/7 and is marketed as a 4K channel, e.g. HBO 4K. I think there was an expectation that that would happen because that's how it happened with HD; over time, HD versions of channels rolled out and replaced the old SD version.
But so much of the high-value entertainment content now being produced isn't destined for linear channels but rather on-demand streaming apps, and typically in 4K. Perhaps we'll see shows and movies airing on cable and broadcast networks also start showing up in 4K too, but if/when that happens, I expect that cable operators will just offer it in that format via their on-demand platform. (DTV and Dish satellite services might put those shows on their few dedicated 4K channels that feature content from a variety of networks.) I know that some recent FX shows have been available in 4K the following day on Hulu. I don't know if cable operators are given the 4K version to distribute to their customers. (Would be weird if not, but who knows.)
Think about the typical cable channel and what percentage of its content is new stuff (which might be produced in 4K) and how much of it is reruns from years past. To launch a dedicated 4K version of the channel would mean going back and rescanning lots of old content into 4K or, more likely, just artificially upscaling all that old HD content into 4K, which is something your 4K TV/cable box can already do pretty well. That would mean cable operators would have to devote a big chunk of bandwidth for a 4K channel that is mostly just airing upscaled HD stuff.
Now, as cable operators dump QAM and switch to managed IPTV, their system should be able to switch the stream sent to viewers from HD to 4K on the fly. So while the cable box/app remains positioned on the same linear channel, the resolution of that channel's stream could increase or decrease depending on the show being aired. Kinda the same way that YouTube works, where different videos in the same auto-playlist will stream to the viewer in different resolutions.