Did you read the release notes, readme.html? First bullet item under Configuration Issues, under JBossAS 4.0.0.GA.
P.S. I hope you installed by unzipping the binary zip file, otherwise you might not have the readme file. In which case look at http://www.jboss.com/index.html?module=bb&op=viewtopic&t=131613.
Actually, after reading that bullet item again, it says that the app server now binds to localhost instead of 0.0.0.0, but it never explains what this means!