Assuming you have the itch and can't wait for better technology in a few years (which is the sensible thing, right?) the recent price drop on the 295x2 would probably make that the easiest "4k-able" drop in replacement for what you have. It'll eat up your single 16x PCIe 3.0 slot but it will give you a 290x CF solution, which is sorta the minimum needed for 4k. Going with two single-GPU cards that are comparable will cost about the same (with today's pricing) Also set your expectations going into it, know that you will turn down some settings like AA to get playable framerates (one might argue you don't need those settings at 4k).
Or you if you have money and don't care about a budget, buy a 3rd card (a 290x) and run in a 16x\8x configuration in Trifire on a board that can support it. that gets expensive quick though.