My suggestion is to first buy your 4k monitor and see how you do.
Some games are graphics limited, and some are cpu core speed limited.
Very few can actually make effective use of more than 4 threads.
4k resolution puts a higher demand on the graphics card, but not necessarily on the cpu to deliver the frames.
Have you overclocked your current I5-4690K?
If not, you should do so.
80% cpu busy is indeed a high number.
Be careful how you view task manager stats.
Windows will spread out cpu processing among all available threads.
With a 12 thread 8700, you should see a lower total utilization.
But, your actual performance will not differ because the individual core speeds will not be different.
Such a move might be good if you play multiplayer games which are the type that can use many threads.
If you go 8th gen, buy a 8700K instead and a z370 motherboard.
Ehen you oc a 8700K you can get some 30% stronger cpu performance.
But... I would not do that.
I would go 9th gen which should launch soon.
At a similar price, you should be able to get a I7-9700K with 8 full cores and likely a better overclock than any 8th gen processor.
What is your case?
If it is a good one with 160mm available for an air cooler, I would go air.
My canned rant on liquid cooling:
------------------------start of rant-------------------
You buy a liquid cooler to be able to extract an extra multiplier or two out of your OC.
How much do you really need?
I do not much like all in one liquid coolers when a good air cooler like a Noctua or phanteks can do the job just as well.
A liquid cooler will be expensive, noisy, less reliable, and will not cool any better
in a well ventilated case.
Liquid cooling is really air cooling, it just puts the heat exchange in a different place.
The orientation of the radiator will cause a problem.
If you orient it to take in cool air from the outside, you will cool the cpu better, but the hot air then circulates inside the case heating up the graphics card and motherboard.
If you orient it to exhaust(which I think is better) , then your cpu cooling will be less effective because it uses pre heated case air.
Past that, A AIO radiator complicates creating a positive pressure filtered cooling setup which can keep your parts clean.
And... I have read too many tales of woe when a liquid cooler leaks.
Google for AIO leaks to see what can happen.
While unlikely, leaks do happen.
I would support an AIO cooler primarily in a space restricted case.
If one puts looks over function, that is a personal thing; not for me though.
-----------------------end of rant--------------------------
Your pc will be quieter, more reliable, and will be cooled equally well with a decent air cooler.