You can't put 8GB modules in these slots. I tried. Apparently the architecture does not support addressing the extra memory in each slot. My iMac would not successfully boot or run for very long before rebooting. So, you have 2GB and 4GB module options, in pairs, among the 4 slots. That should provide 4GB (two 2GBs), 8GB (four 2GBs or two 4GBs), 12GB (pair of each) and 16GB (four 4GBs) options for total RAM.
I'm no expert, but this was my experience.