Any 4K monitor 26" or greater is the same dpi as FHD at 13"
Various examples here https://www.newegg.com/p/pl?d=4k+monitor
A 24" 4K monitor is only 8% better in fact. I would venture to guess it would be basically indistinguishable to the naked eye.
QHD at 13" is 226 ppi is that really a reasonable minimum?
I use a 13" MBP with no UI magnification, but most web pages scaled to around 200%. That keeps the UI nice and compact and leaves a lot of screen space for actual content (about 2170x1523 with my current browser layout). I do occasionally set it to a scaled resolution when someone else has to watch/use the machine as well.
I couldn't agree more, I was given 16:10 monitors for my current job and I'm not sure if I'll ever go back to 16:9 if I can avoid it. Next laptop is a 13" MBP for sure too.