Both during and after installation (meaning while running the USB iso and booting from HDD after) ubuntu will rapidly become unresponsive and eventually hang when my 4k monitor is plugged in. I have 2 monitors, a 1920x1080, and a 4k. Everything works absolutely perfectly if I only use the 1920 monitor (this is where I am working from right now) but the moment the other monitor is plugged in I get problems.

When I cold boot into the system from the small 1920x1080 monitor and then plug in the 4k monitor, the screen on my smaller monitor will glitch before returning to normal a few seconds later, while the 4k monitor never displays an image. The OS begins to bog down, the mouse begins to lag and become less responsive. If I open the display manager I can see that ubuntu has properly detected the 4k monitor and has the correct default settings (resolution, refresh rate, etc.) but attempting to make any changes will result in an immediate crash/hang upon pressing the "apply" button.

If I cold boot ubuntu with only the 4k monitor plugged in, I see purple splash screen and ubuntu logo screens (at the correct resolutions) but the monitor output stops and I see only a black screen before the login screen is reached.

If I cold boot with both monitors plugged in I see the splash screen and logo, there is a brief moment where the small monitor glitches, and then I see only the magenta backdrop of the login screen but there are no UI elements, and This is only seen on the small monitor; the 4k monitor has once again lost input and turned black.

I might be a little out of my depth as far as debugging hardware/software incompatibilities. If there is a fix available that isn't more cumbersome than installing a different distribution I will be very grateful to have it.

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    Are you using an Nvidia card? If so this issue is fixed when you apply the Nvidia proprietary driver instead of the default driver. Probably works with other card brands also to use their own driver. – ArcanaTigris Oct 30 '18 at 12:40

As ArcanaTigris mentioned in the comments, Switching to the NVIDIA driver in the additional drivers panel does fix this issue, however it dramatically increases boot times from under 20 seconds to over a minute.

While this next part is outside the scope of Ubuntu specifically, It may be useful to some readers:

Linux MINT has a version of the nouveau driver that works with my 4k monitor out of the box. In the end I switched to MINT because of the increased boot times incurred from switching to the NVIDIA driver for ubuntu. Mint also suffers from slow boot times if you switch over to the NVIDIA driver.

Manjaro also has a version of the driver that works out of the box but limits screen refresh rates to 30HZ at resolutions larger than 1920x1080. This is what I was trying to use before switching to Ubuntu, but the low refresh rate was awful.

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