There are tons of reviews of the 780Ti running out of VRAM on 4k monitors on games like FarCry, and 290/x did not, search it. Now were talking FarCry, picture what will happen when StarCitizen comes out and all the new games in 2014. If you own a 4k monitor you will have no choice but to lower your settings, and doing that defeats the purpose of owning a high end card.
Show me proof of this tons of reviews that prove the 780ti is running out of vram at 4k and I may just believe you. LOL