4K monitors have 3840×2160 pixels, four times as many as a 1920×1080 monitor (8.29 million versus 2.07 million), 3.6 times the pixels of a 1920×1200 monitor (such as our 24-inch monitor pick), and 2.25 times the pixels of a 2560×1440 monitor (like our 27-inch monitor pick)  It means more clear and sharp images and text. However, gamers may need to $500 to $700 graphics card to play at high quality settings on a 4K monitor.

You should also know that not all of us need 4K monitors: For people with normal computer use but needing space for viewing at two pages side by side, an ultrawide monitor will do. WIth a resolution of 3440×1440, you get 1.3 times as much screen space as a 27-inch screen, all without scaling. 

The best 4K monitor: Dell P2715Q  (27-inch IPS monitor with a 3840×2160 resolution, highly adjustable stand, VESA-compatible for use with a monitor arm,  built-in USB 3.0 hub, single-stream DisplayPort connection that an run the monitor at its full resolution at 60 Hz.)
The second best 4K monitor: ViewSonic XG2700-4K ( factory-calibrated for accurate colors, 27-inch, 3840×2160 IPS monitor, can run at 60 Hz over HDMI if your computer supports HDMI 2.0, supports AMD’s FreeSync for tear-free gaming between 40 and 60 frames per second with a compatible graphics card)
The best 4 K monitor if you are upgrading: BenQ BL3201PH (big enough to run 4K without scaling, helping you avoid quality issues with third-party apps)
The best affordable 4k monitor: Dell P2415Q (24 inch, ideal for people with limited desk space)