Starting from version 2.6.3, JBoss Portal will formally run out of the box only on JBoss AS 4.2.x and JBoss EAP 4.2. This doesn't mean that it will necessary be impossible to run JBoss Portal on earlier version of JBoss AS but no testing will formally be done and manual steps might be required to get the combinations up and running.
Here is the list of known issues for different versions of JBoss Portal and JBoss AS and how to solve them, feel free to update that list. You can also try different combinations if you are facing issues and report your success/failure here. For example the Identity portlet issue reported on 4.0.5 will also exist on 4.0.4 and can probably be fixed the same way.
JBoss Portal 2.6.2 and JBoss AS 4.0.5
Identity Portlets: The new identity portlets require the use of a version of JBoss XB more recent than what is included in JBoss AS 4.0.5. To fix that issue, it's enough to replace the lib: JBOSS_HOME/lib/jboss-xml-bindings.jar by a version of JBoss XB >= 1.0.0.RC7, using this one would work: jboss-xml-binding.jar 1.0.0.GA and (re)start JBoss AS.
JBoss Portal 2.6.2 and JBoss AS 4.0.5 with JDK 1.4
For JDK 1.4, you have to keep using the former user and role portlets (and remove portal-identity.sar). This means that the user portlet for registration of new users won't be available. For details see: Related Bug.
JBoss Portal 2.6.0 and JBoss AS 4.0.4
WSRP issue: Binary distributions of JBoss Portal include the WSRP service which is not automatically deployed with the source distribution. WSRP is built upon the JBoss WS web service stack. As such, it has some additional constraints. In particular, there is a known issue with the version 1.0.0.GA of JBoss WS (bundled with JBoss Application Server 4.0.4.GA) that prevents the complete deployment of JBoss Portal's WSRP service if the user is not online or behind a firewall/proxy. This, in turn, prevents the deployment of JBoss Portal. If you do not need the WSRP service, you can remove the portal-wsrp.sar file from the jboss-portal.sar file. If you'd like to use the WSRP service, the JBoss WS issue has been addressed in version 1.0.2.GA of JBoss WS. Please follow the instructions on how to upgrade JBoss WS