iMac won't recognize old RAM after memory upgrade (2+2+4=6??)

I just installed a 4GB SO-DIMM from Crucial into my 2009 27" iMac (i5 2.66GHz) running OSX 10.6.8. I had read the apple support page that said, "These computers will not start up if only a single DIMM is installed in any bottom slot; these computers should operate normally with a single DIMM installed in any top slot," so I moved the original 2 DIMMs from their original locations (top slots) to the bottom slots, then installed the new DIMM into a top slot. My iMac starts up normally, but it indicates I have only 6GB RAM, not 8. It recognizes the new 4GB DIMM and one of the old ones, but not both. I pulled them out and reinstalled them to make sure they were inserted properly, but I got the same outcome. System Profiler shows 4GB in BANK0/DIMM0, 2GB in BANK0/DIMM1 and both BANK1 slots empty. The specs on the new memory look correct for this iMac, and the apple support page sugests that you don't need to add DIMMs in pairs, so I'm not sure what went wrong. Any ideas are greatly appreciated. Thanks!