Hello,
Is the GTX 780 6Gb capable of 4k resolution ?
 
I can't find the Hdmi spec. It seem that hdmi 1.4 or 2.0 is necessary ?
 
Thank you !

Yes, the GTX 780 6GB card can do 4K, and with the expanded vRAM on the card, it should even do it fairly well. They're going to be tough to find, though, as they went out of production fairly quickly, though I do see them pop up in RX form on Newegg from time to time.
 
EDIT: It's equipped with a 1.4 spec HDMI port, so  you'll have to use DisplayPort or Dual Link DVI for 4k@60hz.

rjohnson11
As far as I am aware there is no 6GB GTX 780 card. The GTX 780 standard spec is 4GB.
 
I can make a recommendation but I need to know what your budget is.

you may be thinking of the 980, the GTX 780 is 3GB standard, and once the GTX 780ti came out, offered a 6GB version: http://www.evga.com/articles/00830/
 
Also as far as I know, you'll need display port to use the GTX 780 at 4k without sacificing stuff like color or refresh rate, as I don't think that card supports the HDMI 2.0 that's needed for 3840x2160x60p with 4:4:4 color

I can answer this for you since I have a 4k tv and had my GTX 780 before I got a Titan X.
 
You can do 4k @ 60hz no problem using the hdmi 1.4 port on the card. However, it will run at 4:2:0 chroma. So text won't be as sharp and there will be some issues with displaying images/text. 
 
If you're using a monitor that has displayport, I highly recommend that. If not, you should have no problem running @ 4k @ 60hz through the hdmi port as long as you don't mind the 4:2:0 limitation. In games, it's not as much of an issue, but for daily tasks like web browsing it can get annoying. 
 
Hope that helps. 

Is 4k 4:4:4 possible on a gtx 780 ?
 
I'm guessing with Displayport and compatible monitor ?