Depends on your financial situation. Money no object? Gaming -only-? Amdlova is right - the AMD R9-295x2.
Water cooled, VERY quiet, and stupidly powerful. About $1500.00
Money is -somewhat- of a concern? The 780TI is very powerful, and very well designed, runs about $700.00-800.00. If you cant afford that then a single R9-290X will be significantly cheaper, and still very powerful - enough so to drive a 4k display. About $500.00 or so.
Nice thing is that upgrading means just buying a secondary one and running it in crossfire/sli if you need more performance down the road.
I have twin R9-270x's myself, and well, they're pretty darn fast in tandem. R9-290x's are even quite a bit more powerful.