On my super high spec machine, running XPsp3, i tried running both CS4 and CS3 at the same time (yeah i know!)
CS3 = no problems at all.
CS4 = couldnt play videos properly (just flashes random? frames). Some audio tracks were messed up/missing. Exported vids all messed up. Occasional crash. (this was CS4s behaviour before i installed CS3 and tried this little experiment.)
Any news about CS4s Service Pack ?? Its OBVIOUSLY in great demand!!