already got a 4k monitor years ago. The main concern for most with a 4k monitor is that they dont have a powerful enough GPU to actually run games at that resolution, unless you are lucky enough to have a 1080 ti or 2080 ti. If I brought another 4k display, I would try to find a smaller one, ideally 14-15 inches as that would provide much better image quaility than say a 28" -40" display. In theory, your eyes cannot see additional pixels beyond approx 300 pixels per inch, this is the point were aliasing / jagged edges should no longer be an issue. In other words, pixel density is a more important concept than resolution alone (at least, for gamers).
If you can't bare using a small 4k monitor, id say wait for affordable 8k monitors and don't buy a monitor / display over 28/29 inches or you wont get the full image quality benefit from upgrading. you would also need a GPU with a very high pixel rate, probably at least 300 GPixel/s (about double the spec of the rtx 2080 ti). I think multi GPU graphics cards will be a necessity for 8k @ 60fps.
HDMI 2.1 is an important upgrade too for refresh rate, but entirely optional.
Still, its nice to be able to sit close to a 4k screen and not notice individual pixels like you often can on lower res screens.