4K through the Mac Pro HDMI port with 10.9.3
Paul Antico sent in this report on his experience with the 10.9.3 update: I just downloaded 10.9.3 and tested it on my 8 core late 2013 "new" Mac Pro D700s and my 5" late 2013 15" Retina Macbook Pro. I have a 40" Seiki 4K display hooked up to the Mac Pro HDMI port. You can now do "retina" on the 4K panel (HIDPI scaling) and it works ok in most situations. This means photos/video/visual content etc. are shown at native 4K resolution on a big 4K monitor or HDMI 4K/UHD TV, but the screen elements and UI remain at a normal size - say what you see on an iMac at 2560x1440 - but much sharper. It's quite nice, and how the Retina Macbook Pros work. It's not a straight 2x scaling (it actually scales to 5120x2880 and scales down to UHD resolution, but from a slight distance it looks as good as the Retina screen does on my Macbook Pro.) FCPX on those mentioned computers supports broadcast monitoring (HD, and 4K) via HDMI right out of the comp...