Yes, you can install CS4 and CS5 at the same time in the same machine on Windows and Mac machine.
I have Fireworks CS3, CS4 and CS5 installed on the same machine all at the same time on Windows and Mac.
You can have bothin CS4 and CS5 installed but Acrobat 9 will not work if you do. And
if you try and install it anew it will tell you that you already have a superior product installed and the install will terminat
Acrobat 9 runs fine for me on Mac OS 10.6. and I still have CS4 running.
For some reason installing CS5 MC took acrobat 9 Pro down for me. I had to uninstall it and reinstall.