When I say comfortable, it has to seem good (enough) value to you! I'd be looking for memory bandwidth when pushing pixels to a 4k screen so I'd be thinking a 280(X) for the 384-bit bus which I would expect to be an advantage in pulling the textures out of the VRAM over the 760/770's 256-bit bus.
It's interesting to see that on AETAaAS's link to guru3D that the lowest cards that were tested not in a multi-GPU setup were the 7970/280X. I reckon you could easily get something playable on one of those - and there are some good deals cropping up now and them on 7970s/280Xs (a decent aftermarket 280X is under £230 and last week that 7970 popped up on scan at under £200)