I'm using an Nvidia GTX 670 with the nvidia proprietary driver 690.
At 30hz All of the fonts look great, in fact everything looks great. However when I turn the refresh rate up to 60hz many of the fonts become blurry and blotchy. I tried using gnome-tweak-tool to adjust the anti-aliasing but no matter what I do the fonts are blurry.
Just a quick note, Im using a 43" Vizio 4K tv and I have the resolution set at 4096x2160. Also, using the same tv as a monitor for my 2105 macbook retina with the same resolution/refresh rate everything looks fine, so I doubt it's the tv.