At least once a day we get an email asking "Can card X do 4K?"
To which there is literally not a single answer. As in, there are MANY answers.
First up, the crucial question is WHICH DISPLAY? And this is crucial because there are already 3 different types of High-resolution displays out there.
5K Dual Cable
4K SST is what most new 4K displays are. SST means "Single Stream Transport". This requires a recent DisplayPort. For these displays, a single DisplayPort connection from a newer (Kepler or later Nvidia) card will give you the full 4K at 60Hz. Anything from the prior Fermi series (GTX4xx through GTX580 and Quadro 4000-6000) will only give you 2560x1440. No amount of driver fiddling or hacking will change this. That is all that these cards can do.
4K MST were the first 4K displays on the market. The Sharp & Asus 321 displays were soon followed by a Dell 321. They all have slight variations on the model number but all have "321" in their name. The Sharp and Asus are exactly the same display while the Dell came a little later and has some refinements but also some limitations. We got the Asus version when Apple announced that these displays were the supported ones for nMP. And what a frustrating experience it has been. And here is where it gets confusing, or at least more confusing. These displays have a "Dual HDMI" mode to do 4K 60Hz, but Apple will likely never have drivers for that so we forget about that mode. That leaves us single HDMI in or Single DP in. The Single HDMI in will always be limited to 30Hz. The DP input is the source of wonder and anger all at once. Many of the cards that CAN'T do 4K on a 4K SST display will actually do 4K on this port, but at 30Hz. And some cards that originally did 4K 60Hz on DP port stopped doing it later due to either OS updates or Firmware updates on the displays. There is a dirty little hack that allows all Kepler and later Nvidia cards with DP output to run these at 60Hz. I will be detailing that in a separate blog post, if you are desperate it is over at MacRumours in the Mac Pro section, it has "1984" in the title.
The above mentioned hack only works in 10.10 or later and didn't work well in 10.10.1 but by 10.10.3 it now works better. It requires running in Kext Dev Mode but is nice and smooth. There will never be a way to run Nvidia cards in MST 60Hz mode for 10.9.
Part of why this mode is so frustrating is the inane and mindless method of switching from one mode to the other. Instead of a "MST or SST" switch, it is the most mind-numbingly obnoxious series of button pushes. The buttons are unseen around the side of the display and are a short row of 8 buttons. You need to do 10 separate pushes on a variety of them to switch from one mode to the other. The display wants to "time out" of being in button pushing mode, sometimes 1 second after you have pushed a button. You must push them in exactly the right order and the topmost button is actually the "on/off" button so you will frequently turn off the display by accident in the midst of switching from one mode to the other, requiring you to turn it on again, wait for it to get back to normal mode, then start the button pushing again. Assuming you get all 10 pushes in the right order, it will switch modes. Sometimes you get what you want, sometimes it scrunches everything over to the left side and leaves the right side totally black.
"MST" means "Multi Stream Transport" and is actually a newer form of DP where 2 separate monitor feeds get sent down the same cable. So the left and right sides of screen are sent as 2 separate displays but then sewn together in the display. AMD cards from the 7xxx series worked for this starting in 10.9.3 or so. But then I updated the firmware from 1.00 to 1.10 on the Asus and it has never worked with AMD cards since. If anyone knows of a way to get the 1.00 firmware I will re-test. Until then, I can no longer use AMD cards on MST 4K. You get 2 desktops next to each other. Frustratingly you can drag something from one to the other and it looks perfect while it crosses, but that is the only way to see it work right. It would take Apple about 30 seconds to fix this, but they won't.
These are great displays, but they only work really well with the nMP, and even it doesn't get a boot screen. And the Dell variant has idiosyncrasies of it's own. I don't have one and can't speak for it.
Finally we come to 5K Dual Cable. Right now this means Dell UP2715K since it is the only 5K available. The nice thing about the Dell is that it has 2 modes, one of which is SST 4K and only requires a single DP cable. The 5K mode requires 2 @ DP cables, and they must go to the same card. So far, this is only supported on Nvidia cards, and you have to run a hack. (listed on this blog further down) With our Kepler EFI cards you will see a lovely 4K boot screen in SST mode. In 5K mode you will sometimes see a boot screen, but not always. The only Kepler cards that can run the 5K at full 5K are ones with 2 DisplayPorts. So a Quadro K5000 and a GTX760 that has 2 DisplayPorts can do it. If you find another Kepler card that has at least 2 @ DisplayPorts, we can most likely flash it for you.
So, to get back to where we started, the answer to "Can it do 4K?" is a long and messy one. The best way to find out for certain is to find someone who is using the same display that you are interested in, so do some Google searches. Keep in mind that sometimes a firmware or OS update will change things, so at the very least find out what OS is reported as working. The words of a independent 3rd party are usually a good indication when some particular combo works. I can only answer for the specific cards and displays we have. Every day there are more and m,ore 4K displays hitting the market. Most now are SST so it is possible that anything we post about the Dell in 4K SST mode will apply to those, but we can't guaranty that.
Currently the HDMI 2.0 spec can run 4K at 60Hz, sadly OS X doesn't have drivers for this yet. When it does, Maxwell Nvidia cards will in theory be able to run 4K 60HZ from HDMI ports. The reason this is important is that MANY 4K TVs only offer HDMI input, and until those drivers come into being, there is no way to drive one of those TVs with a Mac at 60Hz. Last I checked, only Panasonic had a 4K TV with DP. This is something to think about BEFORE the truck drops off a TV that you can't run properly.
And last but not least, how do you verify for CERTAIN that you are getting 4K 60Hz? There is a site that gives you an easy way to verify. Do a Google search for "UFO 60Hz" and it should be the first hit. Basically it is a site that scoots a little UFO across your screen. They will have one at 30Hz and one at 60Hz and they "lock" on your display and figure out what you are running. You will be able to see the 30Hz one hopping and skipping while the 60Hz UFO will smoothly slide across. And for this you don't need a 4K display, any old display will work.
I will revisit this post and clean up typos and poor phrasing. If you have specific questions, please ask.