I'm no expert, but I play one on Level 1 Techs. Most of this is what I have learned from others.
24" is definitely too small and 27" is kinda small for 4K. I couldn't read the tiny text on a 27" 4K monitor in a store. I know that can be adjusted but sometimes, with some programs and games, one can't increase the font scaling of the user interface.
As a size experiment I cut out three pieces of cardboard and taped it to my old 24" inch screen. I found 27" was too small, 40" was too big and 32" was in the Goldilocks zone. I bought a cheap 32" 1080p screen as a temporary solution (text on my 24" was too small for my eyesight). Then I tried the width of a 40" at the height of a 32" = 38" - 21:9 - Ultrawide. That's what I want but they are too expensive ATM ($1500), plus I don't want a curved monitor. One day 'my Precious' will be mine!
Many TechTube reviewers say that QHD (2560 x 1440) resolution is the sweet spot. The resolution is almost as good as 4K in actual use and much less taxing on your GPU. The monitors are cheaper and you wont need to drop another $700 on a (1080Ti ?) GPU in order to push 4K.
I will try to find a 4K monitor like you asked for and a QHD screen that I hope you consider.
Honestly out of all things computers the monitor is the most important and will last the longest by far. I'd just save up a bit more and shell out for a really nice monitor. I went with that 24 inch dell and I'll have it for the next 10 years. It's going to be a awhile before I'm playing games at 1440p 165 hz on ultra settings.