Just a very short but infuriating hint today. Many people seem to be having issues getting full 60hz 4k output from their XPS Laptops to external devices. I've had the same issue, and my 4k external monitor only ever managed to get 30hz from my Dell XPS 17 over HDMI.
The reason for this is one simple BIOS setting.
You can find it under:
BIOS → Display → Direct Graphics Controller Direct Output Mode
Enabling this setting will allow your external devices to talk directly to your graphics card, rather than being passed through Intel's integrated graphics. This will give HDMI, Thunderbolt and (mini) Display Port the full capability of your system.