Just now back from NAB with a chance to check the forums...
The HP DreamColor Z27 actually has some specialized ways to display 3840x2160 or 4096x2160 sources. (Most monitors cannot display these signals - indeed very very few monitors are even designed to accept input signals of a higher resolution than the monitor panel's native resolution.)
The Z27x can display 4K either scaled to fit the entire display or you can use two different methods to show the larger image pixel-for-pixel. The first lets you hop from corner to corner (and the center) of the 4K image. But the second one is even cooler. It allows you to dynamically scroll around the 4K image. A pop-up window appears in the lower right corner showing you the entire 4K image and a box which indicates which region you are looking at in real time. Using the bezel buttons you can scroll around the image in real-time.
4K input is accepted over either DisplayPort or HDMI. I've tested both coming out of a MacBook Pro and MacPro and they work well. The only frustrating thing is that the formats are not listed in the resolutions list in the Displays control panel. We are investigating this with Apple.
Note that the scaling algorithm that is used in the display to show 4K scaled to fit is highly customized for 4K down scale and produces a pretty gorgeous image. Indeed, many of the beta sites reported that from about three feet away they couldn't tell the difference between the scaled output of the Z27x and a true 4K display.
-greg