Im having the exact problem now when I tried to upgrade from 2*2GB to 4*4GB RAMs. Did you find any solution?
When I install 2*4GB (8GB total) rams alone - BEEP infinitely
When I install 3*4GB (12GB total) rams alone - BEEP infinitely
When I install 4*4GB (16GB total) rams alone - BEEP infinitely
But.....
When I install 2*4GB + 1*2GB(old one which came with the iMac) (10GB total) - Boot up fine
When I install 3*4GB + 1*2GB(old one which came with the iMac) (14GB total) - Boot up fine
Im unable to max the memory to 16GB because of this.
And here I am wondering how do I get 16GB to work?