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.