PowerEdge T610 memory upgrade

My client has a single processor PowerEdge T610. It has 4 x 2GB DDR3-1066 dual ranked RDIMM modules installed in channels A2 A5 A3 and A6. We wish to add 2 x 4GB modules. I bought 2 x 4GB DDr3-1066 RDIMM's. When I add the new modules to channels A1 and A4 and start the server at the initial memory configuration step I get a warning message saying channels A1 and A4 are disabled in this mode. Due to time constraints I was unable to reboot again to get the exact message. I read the Dell paper "Installing and Upgrading DDR3 Memory" but still can't be sure what the problem is here. I would appreciate any advice.