I was wondering about this too and the impact on your eyes. For a 39" monitor it's recommended you sit over 5ft away, so the optimal desk environment for such a large monitor will be interesting.
http://www.rtings.com/info/television-size-to-distance-relat...
Too close and the pixels are going to be "big" and too far away and the pixels are going to be invisible.
For example, at 15 feet away the high res would be a waste of money for that screen size, you can't see better than 480 lines at that range.
Extending my arm, I seem to be around 2.5 feet from this screen. Doing the math, higher that 4K at 40 inch screen would be visibly noticeable although its kind of borderline, so 4K should be good enough.