I recently upgraded to V4. I have SBS2003R2, and the first installation attempt was unsuccessful on the server. I had to uninstall both AC and SQL2005 then reinstall them and it went just fine.
Regarding the clients, you are supposed to get an error message that SQLExpress didn't install correctly. It's a glitch but it's supposed to happen. Watch the instructional videos on the installation, and join tells you that the error message is going to pop up. He says to the effect "trust us, the installation went just fine."
