"When in doubt, READ THE DIRECTIONS!"
Not sure why, but I finally got around to reading the xorg.conf manpage. And found out I've been doing it wrong... My second card's Device configs should have been:
Note that the Screen numbers are 0 and 1, exactly the same as for the first card, not 2 and 3, as I was doing. I was getting confused by the fact that later on, in the ServerLayout section, I do
use Screen 2 and Screen 3 to identify them there. Silly me, I assumed the two sections' tags correlated! But the Device section's 'Screen' must be 0 to one less than the number of heads on that device
(or "entity" as the manpage calls it).
So, now I have all four monitors stretched across the desk in one huge Xineramic display!
Only problem I'm having now is Unreal Tournament (perhaps others but it's the only one I've tried so far) - if I have the screens in Xinerama mode, when I run UT the game window appears full-screen on the display I started it, but a blank window with the title "Unreal Tournament 2003" also appears on one of the other displays. And the mouse is nowhere to be found! If I disable Xinerama (of course, getting a separate desktop per display) the game runs properly.