Yeah, a 4k res sounds about right for 3 screens. =)
I would strongly suggest a 780 if you were willing to part with the cash. Considering how close the perofrmance is when comapred to the Titian: (Look at the top 2...)
http://www.videocardbenchmark.net/high_end_gpus.html
I wouldnt advise waiting for the 800 series. There is talk of the 800 series being released as early as Q1 2014, but I bet it'l be later than that personally. =P
A 780 will be able to max out B4 I should think, but at that resolution, I wouldnt expect to see a very high frame rate. You really do need a good SLI setup for that sort of res. If you were willing to go, for instance, 3 670's or 2 780's, then I would be quite confident in saying you could easily hit 60+ FPS at max settings. However this is all speculation due to the fact that the official system requirements have not been released yet.
Just a side note as well, running such a high res will use a large amount of VRAM, and so a minimum of 3gb should be used. if you can afford it, go for 4gb GPU's, as even though 2gb will work, you will ahve to run on lower settings to decrease VRAM usage. =)