i know that prad.de professionally tests monitors. (maybe use google website translator)
I'd consider to also take signal processing time (input lag) into account - a value oftenly confused with reaction time and like no manufacturer tells.
Also take into account that you are investing in a very expensive setup that might get outdated in 1 year:
- You are searching for a setup that can run games at 4k
- maybe at full graphics?
- maybe also at high fps? (144Hz or even 250Hz?)
I guess you would have to buy 2-4 of the newest high end graphics cards to reach it for the current games. In one year the new generaton of GPUs and games comes out and it might n ot be able to run it at the graphics settings you want... But I'm no expert in high end gaming hardware, so I might be wrong
As far as I know, the newest technique introduced includes raytracing - something only the upcoming high end cards (RTX 2080) can handle. My suggestion would be to wait until this technique works with cards cheaper than 500€ and buy a new one every 2nd year.
By the way: Do you know that you have to stick to Nvidia cards when you choose GSync? In the upper price segment there are also monitors supporting both, GSync AND FreeSync.
Addition: A small research about 4k monitors on prad.de available looks like there are no 4k monitors with 144Hz or more available.
I (as well as the testers there) think that a fast monitor is much better for gaming as one with 4K. But the choice is yours