has included specs for the Canon EOS-1Ds Mark IV in addition to the Canon EOS-1Ds Mark III on their Compatible Digital Cameras
page. The two cameras are listed as 156 x 157 x 80mm, 1,385g and 150 x 160 x 80mm, 1,400g respectively. Update:
Gigapan Systems has informed me that the 1Ds Mark IV specs were a typo.