So it turns out that 60Hz 4K is not well supported on SP3 right now. 30Hz works just fine. This is a limitation of the Intel U-series chips and the bandwidth they can support on a single DisplayPort data stream.
I don't think monitors like the Samsung U28D590D or Asus PB287Q support the tiled MST mode over a single cable that others are suggesting as a workaround. You can try going into the monitor on-screen menu and switching between DP1.2 and DP1.1 mode if something like that is available, but this made no difference on the Asus 4K monitor I have. Many of these monitors have a "Picture-by-Picture" mode where you can use an HDMI-cable for one side of the screen and a DisplayPort cable on the other side and each is 1920*2160. I haven't tried this option yet but will give it a shot next week with a docking station. I suspect this will be a painful approach.
Some older screens like the Asus PQ321Q are actually less advanced and rely on stitching two half screens together to make the single 4K image since no timing controllers to do the full bandwidth were available at design-time. However, since these monitors rely on two streams, the SP3 actually does seem to work if you use the Intel 3621 drivers and the full control panel collage mode to stich it together. Here is a guide on how to set that up:
https://communities.intel.com/thread/48023?start=0&tstart=0
I'll see if I can collect all the info on available 4K consumer panels and summarize into an SP3 4K guide for the site.
Personally, if I had $600-$700 to blow on monitors right now, I'd get two nice 1920x1200 IPS panels or a 2560x1440 IPS with some beer money left over.