I use a 28" 4k monitor with ubuntu 14.04 and it works great(with an nvidia card). One issue is that in gnome 3 the UI scaling cannot be set per monitor so that

I use a 28" 4k monitor with ubuntu 14.04 and it works great(with an nvidia card). One issue is that in gnome 3 the UI scaling cannot be set per monitor so that if you are using a mix of HD and 4K it can become a bit annoying. If you're wanting multiple monitors I recommend upgrading all of them(assuming your graphics card has enough grunt).