What exactly could Apple have done with the ATV4??? They don't own the content. And there is nothing apple can do to force content owners' hands. Even if the ATV had 4K capabilities, it still would suffer from a lack of seamless live and On-demand content
They could have done a lot of things, I will list a few. I won't mention the bugs/glitches problem, but just things that could have been included with the ATV4 at launch that would have made it so much better.
1. They shouldn't of had the Siri remote requirement of tvOS apps at launch. This essentially killed the ATV4 as a gaming device.
2. Apple could have included Safari in tvOS. This could have potentially opened up the ATV4 to much more content too.
3. Apple could have insisted on more a uniformed UI with video apps when it comes to control with both the Siri remote, and IR remotes.
4. They could have enabled local search for movies and apps. Also, allow for offline (meanting no internet) viewing of iTunes libraries.
5. You mentioned it, but 4K. For me, this was not a deal killer for buying the ATV4, but I am sure it would have made it more attractive when people were comparing various tv boxes to get. At the price of the ATV4, it should of had 4k.
There are many more things too.
About content, of course Apple couldn't of forced the content owners'/providers' hands, but a deal could have been made. So many other companies are having their own TV plans coming out, Google, Hulu, Direct TV, Sling. Also, this isn't even on my list, because there are so many other things that could have been done to make the ATV4 the go-to tv box, but that is now no longer the case.