What is JBoss 4.0

JBoss 4.0 is an open source application server written in java was started out as an EJB container, and provides implementation of J2EE that relies on the Enterprise Java Beans specification for functionality.

JBoss application server is freely available to download. You can download it here http://www.jboss.org/downloads/index .
You can download it in both form binary or source distribution.