Your experience is interesting because I have Bridge CS3 and Bridge CS4 installed alongside each other on my G5 (10.5.8). They won't run at the same time, but each seems to run fine on its own. Of course I don't use either of them, so I couldn't say if there are any resulting hiccups, but definitely no launch issues or what-not. Curious that the Bridge version with PSE doesn't behave similarly...
