Early Adopter of 4k here. 8K is for the uselessly Rich bleeding edge adopters. 4k is still fresh, but no longer bleeding edge or early now in my opinion.
Short story, there is already some benefit, but not much yet. So unless you are itchy for new tech hold off on going to spring for that new TV just yet.
I Currently have a Samsung 4k UN65KS8000
It was selected for the low input lag and that I could get it for $1500
I Previously used a Visio 4k D55-D2 4k
Both do very well on input lag in PC/Game modes. The Samsung was rought to initially find out how to turn game mode on, but a little bit of digging and you will find it. Visio was straight forward.
I had a AMD Fury X9 and switched to an Nvidia 1080.
I am obviously bound to HDMI so the 30hz only for the Fury was just not cutting it. the 1080 goes to 60hz and it is so much better. Sadly neither of the companies have Sync Tech available on these TV's yet so I still have to work out the ridiculousness of computers not locking to 60hz for v-sync to prevent screen tear. It is now 2017... geez folks! Work it out! Especially you browers!
at 65 inches and 10 feet away from the TV (i use mine like a monitor) 4k is USELESS in desktop mode unless you sit closer like 2-4 feet. I still run 1080 on the desktop itself because of that. In video games, I run 4k when possible. Several games are poorly coded in the GUI so it can often become unreadable, move OFF screen, or just mess up in weird ways. Can very easily see the visible difference in gaming at 4k with some caveats. Even with 1080 card you have to relax the extra effects such as ssao/aa and detail levels to cope, but the difference is very noticeable either way.
However, the quality is still visible up to 10 feet away. That said, the benefits are a mixed bag. Content of course for 4k is still very lacking. You can always try gimmicky up scaling with mixed results but source really needs to be 4k origin to really see the benefits. Any 4k source will look visibly better on a decent TV no matter how you slice it, just mind those gotchas if you do any gaming at all!
I have given up on consoles so I have no experience on the 4k Scene there.