Is Ubuntu 4K-ready?

since I've investigated quite a bit about the topic recently, I think I ought to answer.

The short answer is: Ubuntu 16.04 with Unity is ready for 4k, with many limitations and various hardware restrictions.

If you want to stay sure you won't get serious issues, I'd go for a desktop computer with a single 4k monitor; make sure you install a recent Nvidia GPU with DP1.2 output. I have personally tested this setup with an Nvidia GTX 750, binary nvidia driver, DP output, single Philips BDM4065UC 4k monitor, and it works totally fine 3840x2160@60Hz ; you can pick 1.5 or 2.0 monitor scaling (aka 'retina adjustment'), depending on your taste. I've found everything was smooth and working without issues.

What I've found much harder to work with is:

• multiple monitors: especially if they're different and especially if you'd like a different scaling on different monitors; dragging windows around such monitors creates strange bugs. Maybe with identical 4k monitors the situation is better, but I wasn't able to test such scenario.
• laptops: I've yet to find a recent laptop working properly with an external 4k monitor (see Which laptop for Ubuntu 16.04 with 4k external monitor?), but many have succeeded at working properly with internal 4k monitors (e.g. I've heard about success with Dell XPS 9350 QHD).

UPDATE FOR 18.04

I'd say that stock 18.04 is a bit less 4k ready than 16.04. I suppose that Unity worked a bit better than Gnome with HiDpi. In 18.04, you can only choose between 200% and 300% scaling, scaled images look blurry, and some apps will just fail to pickup the scaling setting; also, the desktop feels quite laggy when scaling is enabled, even when using a dedicated GPU (both NVidia and AMD graphics was tested), and most window menus and icons will look blurred or pixelated when using scaling. A step back.

On the other hand, AMD GPUs seem to work out of the box with no proprietary driver. I tested RX550 and RX560, they worked properly with a LG 43UD79B monitor.

This german article says that it was running without issues. The author describes that he was changing the scaling factor under the system preferences and that the scaling was set systemwide for all used fonts.

A unconvenience was the size of the mousecursor, which was corrected in /etc/X11/Xresources/x11-common by modifying Xcursor*size: 48.

He also describes a problem when the system goes into energy saving mode, that it doesn't come back on, but this seems to be an issue with Display Port

In libre office, gimp and firefox the author changed the scaling within the program to a fitting scaling size.

I have brought a machine with 4k display(Dell inspiron 7548). I got it with windows fully featured with touch screen. But i am pure techie, and my requirement was either ubuntu or centos. So i tried ubuntu 16.04 and its working fine with most of features.

Plus point :

• Screen looks awesome with 4k display
• Touch Screen is working fine but not full functional.

Minus :

• Screen rendering is not good.
• Not able to play videos in full screen mode.
• Could not open images over facebook.

it depends entirely upon your graphics card and firmware. I have a geforce gtx 750ii card with the nvidia driver from the ubuntu repositories. excellent 4k experience for videos and gaming.

Ubuntu 20.04 LTS is more than 4K ready. I see no blurry icons even on lots of proprietary apps (unlike Xubuntu or Manjaro, which have horrible 4K support). Application window frames are properly scaled. Applications detect the scaling well, too. Haven't seen an unscaled app yet.

Be aware of the performance it requires though. I have a laptop with Intel i7 CPU with built-in graphics and I can tell you that web browsing whether on Firefox or Chrome, is definitely not fluent. Firefox's performance is a bit better though.