IMO now is not a good time to jump on the 4K wagon.
Firstly the cost on graphic cards require to game on 4K comfortably simply cost too much...ideally you'd want 3 high-end card at least instead of 2, and at which point when there's enough GPU grunt, the 4GB vram might become not enough.
Also considering from the pricing for 4K monitor...like the 4K TV, the price was quite high at the beginning, but they quickly fallen due to not enough people buying, and too many manufacturers launching too many 4K products. If you wait till same time next year, most likely not only you would be able to get two cards at the same price that would have more vram that can deliver the same performance if 3 high-end cards right now...not only that, the chances are with more 4K monitors on the market, the average pricing of the would probably fall even lower- they are may be £450-£550 on average right now, but next year they may probably be in £300-£350 range.