i got a acer x34p, its not bad. there is also a 38" ultrawide from lg but it costs around 2000$.
btw: there is no hardware that gives you good frames at 4k, 60fps max.
More info is needed.
What size? (personally I think anything below 29" is insufficient for 4K and you'd be better off with 1440p.
What refresh rate? Are you happy with 60Hz or do you plan to push more FPS?
HDR or calibrated colours?
Freesync or G-sync? (depending on whether you run nVidia card or AMD. (Freesync works on both but nVidia is really picky and only works on some specified Freesync monitors)
Go look at monitors before buying. There is no way around it. The difference between screens can be very big even tho they have the same specs. If you have poor or good eyesight is also a thing to consider. 4k screens have few value if your eyesight is bad.
Try and decide on resolution and aspect ratio. Also determine your budget. Then visit some shops. You can always buy the goods online for the best price.