Why not both? If I'm on linux, with no interest in changing and perfectly happy with my display, and 4k doesn't work easily on my system, why would I be interested in a 4k screen?

Strange. I'm not seeing any issues with Linux and 4k. I'm running a plain Debian 10 with OpenBox running on 4x 4k (3x 28" in a row and one 11" 4k under the right-most) monitors though, granted, I only normally have one web browser that follows me around, across work spaces pinned to the right monitor, mostly maximized Sublime on the middle monitor and a pile pile of alacritty/xterm windows on the left-most monitor. The small monitor which content also follows me around contains clipboard, clocks, Slack and monitoring.

What is the software that people are using that creates problems?

So far, I've never had an issue with KDE Plasma and 4K@60Hz on linux, once I realized that you can't just use any old HDMI cable: you need DisplayPort or HDMI2

Its a problem if you have a nvidia gpu (forcing you on x11) and have 2 different dpi screens. On AMD/Intel/Nvidia open source drivers it works fine.

Yeah, I’ve always picked my GPU to work well on Linux, so I have an AMD one.