I would not class the 290X as a 4K card, 4gb of vram is not enough to max games. To do that you are going to need three or four 6gb Titans and even then you are at the mercy of driver support.
I know AMD keep on about the 290X being up to the job but when it is demoed at 4K, it is with the settings turned down or using games that are not very demanding. This rather defeats the point of 4K.