I was struggling with WoW. If I enabled Larger Text in windows (because everything is too tiny at 4K), mouse look in WoW would go berserk -- but only mouse look. The mouse itself was fine. Blizzard claims their games are not compatible with text enlargement. This is going to become more of a problem as larger displays become more mainstream.
Some videos on the internet suggested enabling compatibility mode on the wow-64.exe would fix it, but that did not work for me. Fortunately, I was able to get it to work by disabling display scaling.
I fired up Deus Ex:HR with the same settings I had before - 1920x1080 Fullscreen Windowed, and I thought it looked... fine. I didn't think it looked any worse than it did on my 1080p monitor. Started a New Game+ and when I finally got to the Dubai drop-in I still thought it looked fine, including the text in the inventory and menus. Then I cranked it to 4K, and it looked stunningly awesome. My GTX970 was giving my 25fps here. Then I lowered it back to 1080p and, yeah, it looked terrible compared to 4K.
I played some Overwatch at 1080p Fullscreen Windowed and it also looked great, getting 70+ FPS.
WoW might be the outlier. Unlike in Deus Ex and Overwatch, you're not given a resolution option in Fullscreen Windowed, You have to run at the desktop resolution. To lower the resolution in WoW you have to chose "Windowed", which means the monitor (or GPU) is doing the down-scaling instead of the game. And it does not look good at 1080p Windowed, not as good as my 1080p monitor. It might also be because WoW has so much text -- the chat windows, the action bars, the item levels on every item in your inventory, etc. The blurring is more noticeable.
WoW at 4K gives me 40-60fps (on High). I plan to keep running this for a week and then decide what I'm going to do with the 4K monitor. I also got some eyestrain last night, which is very concerning, as I never get eyestrain.