When I moved away from a lower resolution dual monitor display to a single 4k display my current video card's display port could only display 4k at 30Hz which made the mouse feel continually laggy. To get 4k resolution at 60Hz a video card with a 1.2 version display port was needed. After considerable searching for a linux compatible 4k video card at a low price I decided to go with a Sapphire Radeon RX 460 for $120. The card was a painless upgrade and worked out of the box without loading any special drives into a stock Ubuntu installation. I was also surprised to find the video card didn't require a dedicated power connection which is normally required on mid to high-end video cards.
Although the card worked out of the box the system was burdened and ran sluggishly. System Monitor showed the compiz process using a higher than normal amount of CPU usage which was resolved by installing the AMDGPU-PRO Driver for Linux.