My understanding was that Xinerama only affected acceleration with the open drivers, but AFAIK even that is no longer the case with the latest open stack. That is only vague recollection though, don't take it as fact.
I finally got all 4 screens to light up, but couldn't convince the two monitors on the second card not to be clones. Where is the 'clone'/multiple desktop setting kept?
Also, there is a known issue with xinerama in xorg below 1.8.1 which causes it to lock up when the mouse pointer traverses screen boundaries in certain circumstances. I tried to update to xorg 18.104.22.1685 (aka 1.9.0 RC5), but fglrx fails to load due to undefined symbol 'savedScreenInfo'. Any news on that?