Okay so i took both your screenshots and set them to 100% zoom. I then downsampled my screen to match your resolution and took a few screen shots. Then i put both screenshots at 100% zoom and compared them side by side. At first i could not tell a difference so i zoomed into about 160% on both pictures side by side. On my downsampled picture there is a very distinct and definable pixel blur that occurs as you zoom closer and closer. Your picture however doesn't seem to have this sort of blur.
Next i put them both in a generic picture viewer(paint) to make sure there is no software enhance zoom aliasing. After zooming as close as possible on a common image(the cancel button) your can see distinct differences in my downsampled picture vs yours. Namely you can see in mine a slight color inaccuracy and an overclean pixel distinction(something common in downsampling), but on yours it is just the same as a picture of a native screenshot(or at least VERY close to it). With this test my only conclusion is that you are either running at 4K natively OR that monitor is using a much better pixel compression method than downsampling(doubtful as it was have to be something extra internal in the panel but is a possibility, either way it doesn't matter since the clarity would be visually identical aside from an almost forensic test).
Here are the pics hopefully they post well directly for your comparison.
4K seemingly native(your screen)
4K downsampled(my screen)
As you can see even if it is not 4K native you are doing a lot better than 4K downsampled even to a point that i don't think anyone could tell if it was right next to a true native 4K screen. Maybe the koreans are holding out on us and every once in a while a screen gets mis-labeled as 1440p when it is actually some un-released 27" 4K screen. I wish the other user who had a screen that performed this way would chim in and post some screen shots. Bottom line is that you are NOT downsampling. There is either some other aliasing method going on or it is a native 4K screen.
EDIT: This does kinda confirm that a Dual link DVI connection can support 4K@55hz+ whether it is downsampled or native(basically the overall data rate is supported). This is kinda useful in some of those unlocked 4K TV's if they only had a DVI port on them
EDIT2: All i can say is enjoy your 4K 27" monitor. Hopefully anybody else who buys this panel will see this thread and see if this is repeatable. If it is then that would make this one of the best panels on the market in terms of density. Heck if we got 2-3 more positive reports of this doing 4K seemingly natively i would buy one.Edited by Spartan F8 - 10/19/13 at 1:19pm