HDCP Issues with HD content

I have just setup a new machine with Windows 8.1, an Nvidia Titan X (supports HDCP) connected via a display-port cable (supports HDCP) to a Dell P2415q 4K screen (supports HDCP), yet if I try to watch any of my HD content, iTunes refuses claiming I need to use a HDCP display.

I am using the latest drivers, and a fully updated windows 8.1 install, so as far as I can tell, the HDCP chain should be complete and working, so there is no reason that I should be seeing these issues, and I should be able to watch my content.