Well, if you can stretch to $400 I’d personally opt for a BenQ BL2711U 27-Inch IPS 4K Monitor:-

https://www.amazon.com/dp/B00RORBPEW/?tag=thewire06-20&linkCode=xm2&ascsubtag=AgEAAAAAAAAAAPQL&th=1

I would go into a nearby store though and see for yourself first - as to whether you can tell the difference between a 4K monitor and a full HD monitor, or a 1440 monitor etc. And also think about what would you actually be using it for - gaming, video work, watching films or just general computer use… (and questions like will it fit on your desk, what outputs does your pc have, do you want an LCD or LED - or not bothered by it? etc)

Also review sites may be of help - a lot of stats and expect the odd negative review - though should be able to point you in the right direction. Whatever you do decide - have a good look around the net - and also in store to find a better deal if possible.

You may wish to give some consideration to the Dell 4K S2817Q 28" Screen LCD Monitor - available for at for $330 new, too:-

https://www.amazon.com/Dell-S2817Q-Screen-LCD-Monitor/dp/B01BBOC16O

I’ve just upgraded my monitor to a 27" AOC monitor (luckily got it free as 2nd hand from a friend), just in HD res - nothing really too fancy, but a noticeable difference than my old Hanspree 19" - even for more middle-age eyes! 😃

Good luck with it 😃