When I had my 7800GT in a couple months ago, I noticed that when I had SS already on and when I added 4xAA on top of everything else, the game would keep the 4xAA but turn the SS off. However, and here's the kicker, the SS still said it was on. I can tell the difference between FEAR's regular shadows and SS's and I remember double checking everything over and over again until I realized that it just left the setting for SS to "On" when indeed it wasn't.
When I did finally get the SS's to work with 4xAA as well, the shadows looked funky and performance was crawling.
I'm guessing when you apply 4xAA and SS's, it automatically keeps the 4xAA applied, turns off the SS's, but still displays the SS's setting as being, "On". At least, that's what happened to me using your same drivers, 91.31.