I've done a search first and no one else seems to have got any reply from channel 4 (unless you names it something I couldn't search for like C4!). I emailed asking about any plans they had for HD (mainly E4 though) and got this reply:
Thank you for contacting Channel 4 Viewer Enquiries regarding High
We are currently testing this platform, and hope to have more information on
when this may be available in the coming months.
Thank you again for taking the time to contact us here at Channel 4, we hope
you continue to enjoy our programming.
Channel 4 Viewer Enquiries