Just received my Dell 2415q 4k monitor. I plugged it in to my MBPr 13" Late 2013) using the supplied display port to mini display port cable via thunderbolt 2 port. Opened up Switch ResX and entered a custom resolution of 450 pixel clock, and set the resolution to 3840x2160. saved and rebooted. I'm pretty stoked that this worked, my little MBP is driving this new monitor at HDPI 1080p @ 51Mz.