DPI is the wrong measure. What matters is dots per degree. If the device is 2x further way, it can have half the DPI. At typical viewing distances, my phone covers about 1/3rd the fov of my monitor, despite having being a 16x smaller screen. As such, 4k is totally sufficient for pretty much all screen sizes. If the screen gets bigger, you get further away.
Yes and no. For laptop screen sizes (15" and less), 4k is currently pretty good for text quality. However, for ~27" monitors, even accounting for a greater viewing distance, we're still a bit short.
My 14" laptop has the same pixel count as my 27" monitor. Sure I keep my laptop closer to my face normally, but I'm not keeping my 27" far enough to have comparable density (for my current viewing distance, it should be more than double to match the dots/degree).
That being said, I cannot reiterate how much difference this makes for text quality despite still not being ideal.
I once tried an 8K monitor and it was amazing how big a difference it made. The tiniest differences in fonts were easily visible. It was like a high quality glossy print that could move and update.
Since Apple removed subpixel AA everywhere, by default, unless you re-enable it you get absolutely gorgeous text rendering on the internal screen but as soon as you plug in a non-hidpi external screen (such as my 34” widescreen which has a 50% lower physical resolution than the laptop) it devolves into a blurry and barely readable mess.
