I have a 4k 34in monitor and its great. I also have a 4k 17in laptop screen. The text on that laptop screen is super tiny at 100% zoom, but its still sharp and crisp. It takes some training for your eyes to read that from 12 inches away. I also have super thick glasses for my spectacularly bad myopia, but I can read those screens where people with 20/20 vision cannot.
For years I had optometrists tell me that I'd be facing presbyopia soon and after a decade of hearing about I finally got it. Because I am both a jock and a nerd I was advised to have two sets of glasses. I have great vision for detail on screens now and particularly enjoy the 3D effects on the Nintendo 3DS with reading glasses.
Now the 4K screen on my "gaming laptop" is beautiful for ordinary use and sucks for games.
I found I would just get hit by people in League of Legends and it seemed there was nothing I could do about it. I hooked up a monitor and displayed a clock on the screen and got a photo that said (something like) 1:05:03.000 on on the laptop screen and 1:05:03.030 on the attached monitor, an extra 30 ms of latency. I switched to an external monitor and started to win.
Most people claim that display latency doesn't matter for 1-player games but I've found there are many titles like Sword Art Online: Fatal Bullet where I struggle on my Samsung TV when it isn't in game mode.
I have a 4k 34in monitor and its great. I also have a 4k 17in laptop screen. The text on that laptop screen is super tiny at 100% zoom, but its still sharp and crisp. It takes some training for your eyes to read that from 12 inches away. I also have super thick glasses for my spectacularly bad myopia, but I can read those screens where people with 20/20 vision cannot.