The new rMBP comes with thunderbolt 2, a port that according to Intel can drive a 4k display. Can anyone assure me that the rMBP 13" would actually be able to drive such a display, and if so how many?
According to the information we have so far, no, the new Haswell rMBP will not drive a 4k display via TB2.
You might be able to connect it using a HDMI <-> Displayport connector, but since the rMBP only supports 4k at a 24 Hz refresh rate, you will notice significant judder.