True but not really relevant. What does that suggested distance even mean?
Part of it is to do with the physical amount of the screen you can actually see given the field of view and field of focus of the human eye. But I got my numbers wrong, for a 4K display it's actually a little over four feet for a 43" display.
When did people start obsessing about pixel density anyway? 22-23" gave you less density yet no one was really complaining back when 1080p was the standard.
Pixel density and fidelity sort of go hand in hand. I'm not sure I see the logic in quadrupling the size of the screen, with all the implications that has, for no actual gain in fidelity. From the optimal viewing distances and all else such as panel quality being equal, a 1080p monitor and a 4k monitor with the same pixel density will look exactly the same. It's just the latter will be massively more costly to buy and drive; in fact in order to get the same frame performance from the same pixel density the 4k monitor will actually look worse
because it will require sacrifices elsewhere in performance.
Smaller 4K monitors with larger pixel densities are almost useless for general productivity
They're absolutely not; 34-38" is widely accepted as the "sweet spot" in size and pixel density for 4K monitors. Hence pretty much every single professional-grade 4K monitor on the market coming in that size range. We've got some at work; the pixel densities are similar to 25-27" 2K screens which is again the "sweet spot" for professional grade 2K monitors. The reason you're struggling to find many options meeting your requirements is becasue they're very, very niche.
(scaling sucks and it drains your GPU resources)
These are also problems for larger monitors, particularly the latter.
Plenty of PPI and thus it should provide text that's about as readable as it is on a typical 1080p screen.
I know it's a very subjective thing but I've never had an issue with 110PPI, which is the density of a 2K 27" monitor. It's what, 10% more than the pixel density of the monitor you're proposing.
I'm not trying to discourage you or anything; people have different interests and spend their money on different things, but it strike me as a serious amount of money to spend, and performance hit to take, for exactly no tangible benefit.