Thank you for your quick answer.
I'm not a gamer.
I'm a little disappointed.
The GTX760 is not an exceptional graphic card.
I thought GTX 680 or 980 needed to get 4K@60hz.
You have no problem with, blurry ?
Do you need an extra electrical connexion ?
I am a screen real estate junky. I just like to have lots of windows open and visible at the same time. (Using Divvy to arrange the windows nicely.) I upgraded from 2x19" 4:3, and 1x23" 16:10 screen to this so that I could free up desk space (Yeah, I got a 40" screen to free up desk space!) while not sacrificing screen real estate. I had been looking at the 34" ultrawides, but the screen space to price ratio put me off.
If you are not going to game then you don't need a particularly powerful video card to drive 4K. My display is perfectly clear and sharp with display preferences set to "Default for Display" 3840x2160. And scrolling and navigating around OS X and applications is smooth.
The only gaming I have done is a bit of Diablo 3. At 2560x1440 it looked great, and ran fine on my GTX760. 3840x2160 looked awesome, but it was a bit jerky just walking around the camp, so I didn't try any actual game play. You would need a better card to play at 4K or more demanding games.
The ASUS GTX 760 has a single 8 pin power connector, and comes with a 1x 8 pin to 2x 6 pin adapter, which I have connected the 2x6 pin on board supplies to. So no auxiliary power supplies are needed. It has been running like this for 6 months or so with no problems. Many of the GTX 760s I looked at had a 6 pin + 8 pin setup, so I avoided those cards to be safe.
But I bought the GTX 760 about 6 months ago, if I was buying now I would definably look at one of the maxwell cards, 960, 970, or 980 depending on budget and if you want to play games. Just make sure you get one that has 2x6 pin power connectors max so you can run it safely on internal power. I may grab a 970 or 980 at some point, depending on how my gaming habits develop, but the 760 is more than adequate for non gaming uses.