Really depends whether you want to play ALL games at 4k 60, or are OK with just being able to play most of them that way. I tried 4K 60 with a fairly high end TV (49" Sony X900E). At the time, as is still the case today, the most powerful GPU was the GTX 1080 Ti. I bought what was claimed to be the best model at that time, the EVGA FTW3.
Battlefield 1 played quite well at 4K 60, but there were a few games like Ghost Recon Wildlands, and even older games like Far Cry 4 and Just Cause 3 that would only yield 45 FPS or so. Plus HDR is a work in progress, especially with an Nvidia GPU because Nvidia uses Microsoft's HDR in W10, and it's broken.
You have to enable HDR in W10 before you launch a game that supports HDR, and as soon as you do it makes the W10 desktop all washed out. And it's even confusing and quirky in game too. It's a guess as to whether you need to enable or disable HDR in the game, and even if it does enable with an obvious visual difference, it can crash the game.
That was the case with Mass Effect Andromeda for me. The visuals looked great once I enabled HDR in W10, but disabled in game, but as soon as I hit Esc, the game would crash every time. For all I know this may have been fixed by now with GPU driver updates or W10 updates, but I still see threads of people complaining about HDR washing out the W10 desktop.
I also had problems with using 4k desktop res. The default font scaling allows you to see fonts reasonably well, but can make some program GUIs to be scrunched and unreadable. You can solve it by choosing less fonts scaling in W10, but that makes fonts too small to read easily.
Lastly, one of the reason I returned the 4K TV and 1080 Ti is I was also using the TV for TV broadcasts, and the reality is, there is no 4K TV, not even in a smaller 43" size, that shows 1080 broadcasts as well as a 1080p TV can. The Sonys are supposed to have the best upconverting and scaling, but in most broadcasts they only upconvert certain things.
For instance a football game will show close ups of players extremely well, but the field and stadium are blurry, and when they show distant shots of both teams, everything is blurry. Now of course none of this applies if you plan on going with a 4k monitor instead of TV, but my advice on 4K TVs is they are not going to be practical until UHD broadcasts are prevalent, which will not be for at least another 5-6 years probably.
As for what it takes for 4K 60 going forward though, that is hard to tell since we haven't even seen the new Nvidia and AMD GPUs. A lot of people have touted the 1080 Ti as such, but since there's a handful of games it will only play at 4K 45, and we are talking planning for at least 3 or more years of use, the only way you can currently be sure of having 4K 60 is with SLI, and unfortunately SLI is not supported as much as it used to be by developers.