I tested a little intel, amd and nvidia gpus in 4k linux desktop and after having issues with video tearing in videos and small issues I chose the nvidia since It worked the best for my settings.
I wanted to recommend amd because of its generally good open source drivers, but I having the issues with getting 4k@60fps on hdmi output without using a displayport to hdmi adapter as mentioned here. I suspect that I would not have these if my monitor was displayport (and this is possibly a issue that might be fixable in the future), but I can't confirm since I don't have a displayport monitor. If you can confirm this before buying and you intent to buy a displayport the amd might be also a good alternative due to is opensouce drivers.