Just picked up a LG 4k display (27EA83R
LG Display at Newegg) to go along with my new rMBP. I have the late 2013 15" with a 2.3 i7 and NVIDIA 750M. I connected it using a mini displayport to displayport cable and perfect - 2560 x 1440 @ 60 Hz. Apple never documented this would work, so I took a chance assuming it would and perfect! I'm surprised Apple doesn't promote this capability. FAR better than the 2560 x 1440 @ 30 Hz they document can be achieved using the HDMI port. It also works perfectly when booted to my BootCamp partition running Windows 7 64 bit.
There you go - for anyone interested, tested and working. Run out and buy one! Or even two, why not? They look great!