I've been running ArchLinux on my X1 Carbon with some success now; I'll try to use this stream as a place to write down what I've tried, and what I've found to work.
I have a Dell P2715Q 4k monitor that works with the miniDisplayPort port directly at 4k resolution with 60hz refresh.
However, if I connect via the DisplayPort port on the OneLink adapter, X only lets me use the monitor at 4k with 30Hz, or a lower resolution if I want 60Hz.
This is with Xorg Server 1.17.2, and kernel 4.1.6-1-ARCH from ArchLinux.
The webcam is functional through xawtv -c /dev/video0, though I've not yet tried it with a video chat application.