Channel 4 HD is available for free on Sky HD and from today Freesat HD recievers.
YOu may need to perform a rescan of the channels.
YOu will find Channel 4 HD on:
Sky channel 140
Freesat channel 126
Since launch on satellite a few years ago, C4HD was a Sky exclusive channels.
This brings the number of HD channels on Freesat to 4 : BBC1HD, BBCHD, ITV1HD and C4HD.
And soon Japanese new channel NHK World will also have a HD version available on both Sky HD and Freesat HD receivers.