I just got a 4K 27" HE Envy monitor and have a 2015 rMB. I would have assumed i would have been able to output at 3840 x 2160 over the USB-C monitor input which also charges the rMB. However, I can only get the output to 2560x1600. Maybe that was a bad assumption. Apple says the device can drive a 3840 4K using HDMI via a USB-C dongle. Why would it not also drive at 3840 via USB-C directly? Do i really need a HDMI dongle to connect to a USB-C monitor?