All you need to do for 4K resolution is use the HDMI port. At least from the specs. I don't have a 4K monitor, so I can't test it.
The 680 is a great card for that. The 690 can only be used 50%, because it is a dual GPU card and PR can only use one GPU, so is a waste of money for PR and your intended use.
Wish I could test it - but I am in the same club as Harm - no access to 4K monitors.
Yet if it says so in the specs, I have no reason not to believe them.
On a side note: if the 4K panel you are planning to get supports 10-bit color, and you want that support, go with Quadro K5000.
4K support for HDMi is part of the HDMI1.4 standard. Since HDMI is standardized, in order for Nvidia to state they have HDMI1.4 support then their HDMi ports have to support that resolution. So yes the HDMI will work for 4K preview.