./cgminer -k diablo -v 1 -w 256 -I 7 (Diablo Kernel, worksize 256, Vectors 1, Intensity 7)
This might sound stupid, but try adding a variable for all 4 of your GPUs. For example:
./cgminer -k diablo,diablo,diablo,diablo -v 1,1,1,1 -w 256,256,256,256 -I 7,7,7,7 --gpu-mediff -150
I've tried this and it doesn't work either. Thanks for the suggestion though, it was worth trying.
I played around with Xubuntu some more last night (again, everything runs fine on Windows, but I prefer Linux), and still no luck. I'm surprised there are more than just a few having this issue, and no one's resolved it yet. The only thing left to try is a different, non-Ubuntu (or Debian-based) Linux distro, as that would help us tell if it's either just some bad packages or if it's the Linux drivers themselves.