Hello,
I have the dilemma of buying a 4K monitor, or 2 FHD monitors. I prefer the 4K for real estate reasons, but I am reading conflicting stories about the HiDPI support on KDE.
90% of my time is spent on text editors, Konsole, Firefox and Libreoffice, so, I need to know how good those applications are when used with a 4K monitor before I move to my purchase.
My distro is Fedora, currently 31 and will be upgraded to 32 around the time I will have the monitor(s).

Thank you in advance

A 4K monitor for Text editing and Browsing is like buying a Ferrari to go shopping. Unless you really are into videos and gaming most of the time it is not worth the money. Keep in mind that with any HiDPI monitor you will have to adjust the scaling if you want to be able to actually read anything.
I seriously doubt there are free desktops out there that are better at supporting HiDPI, don't let contradicting reports fool you. Currently the only decent 4K support you get are in proprietary systems because, ah yes, proprietary drivers and their sketchy support of Free Software.
Having a 4K capable monitor is not enough, you would also need a proper GPU for this, capable of running games in at least 2K, so unless you are already on the state of the art on that side, again, not worth the money.
Did I mention you also need HDMI 2.0 support on your computer and proper cables? Else there is only Display Port bringing in enough power to run such a monitor.
Ye, it is tempting, "think of the future" and all, but you need hardware capable of supporting this on both sides so the bill might get a bit heavier than expected.

As of now, I wouldn't want a 4K, and waiting to get better support will also bring down prices considerably on all fronts (except GPU, of course).

DP or HDMI 2.0 is not is not an issue. Price is not an issue either. Going for a dual FHD setup costs around 500€, and an entry level 4K is around this price tag too.
My issue is that I don't have the proper space for a dual monitor (left-right) setup. In any case, it looks like it's not an option

tpe: I'm running a 43" 4k TV as a monitor. This thing is large enough that I don't normally have to worry about scaling programs up. For reasons of science, I forced 192 DPI then opened up Firefox, Konsole, and Libre Writer to simulate a smaller 4k display. I'm impressed with how everything looks. The contents of dialog boxes are consistent with their dimensions. I think a 4k display would work well for you.

mamarok: Larger 4k monitors are still overpriced but most televisions made in the last 2-3 years will run 4k @ 60Hz @ 4:4:4 chroma and they work fine as monitors. The TCL I'm using now cost ~250USD. High-end video cards are no longer needed, either. Built-in graphics from Haswell and Broadwell forward (above i3) will run 4k. Today even the 10 watt Gemini Lake platforms will run dual 4k and capable discrete cards exist for under 100USD.