My old Mac Pro works fine with my LG 4K TV under both Mavericks and Yosemite.
Anyway, did you try to manually set the resolution to 4K? I think the system will give you 1080 HiDPI by default. Which actually make sense. Even though my TV is 84", use native 4K resolution still make all icons and text looks quite small if I am at the "optimum" distance.
Anyway, the setting is in system preferences -> display -> scaled. If no 3840x2160 there, you can hold down the option key before you click "scaled". That should give you all available resolution to choose.