./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