I have 1920x1440 in a 19" screen.
Sounds lovely - is it a CRT? If not what's it called? (A few years back you could get 1600x1200 LCDs but everything has become letterbox since then.)
Also, you should know that your claims about driving 3840x2160 are a bit misleading. You may be able to do it over a single HDMI link, but you're not getting full 60 Hz progressive refresh. HDMI doesn't have enough bandwidth to support that. You're probably dropping down to ~30 or even ~15 Hz refresh.
That's what I suspected too. Usually there is an on-screen display which shows you the refresh rate you're using.
Cool. I have monitors with a similar resolution but only 22 inches - so I have to set the font size to extra large in every application. I imagine that you can keep fonts at 'normal' size and just have a super huge desktop. Here's a photo of my setup: http://farm8.staticflickr.com/7145/6851350945_e582af1ed5_o.jpg (it's a big image but I guess you will have no problem displaying it!) Do you have a photo of yours?
The goal of science is to build better mousetraps. The goal of nature is to build better mice.