Have you seen http://msdn.demoservers.com? It's simple — no complex setup or installation is required to try out BizTalk 2004 running in the full-featured MSDN Virtual Lab. As part of the MSDN Virtual Lab, you will have full access to BizTalk 2004 through the following modules:
Microsoft BizTalk 2004 Server 2004 - Creating Schemas
Microsoft BizTalk 2004 Server 2004 - Creating a Schema Map
Microsoft BizTalk 2004 Server 2004 - Enabling Message Routing
Microsoft BizTalk 2004 Server 2004 - Creating a Custom Encryption Pipeline
Microsoft BizTalk 2004 Server 2004 - Creating a Deployment Package
Microsoft BizTalk 2004 Server 2004 - Tracking and Debugging Messages
Microsoft BizTalk 2004 Server 2004 - Creating and Debugging an Orchestration
Microsoft BizTalk 2004 Server 2004 - Working with Orchestration Shapes
Microsoft BizTalk 2004 Server 2004 - Creating Transactions
Microsoft BizTalk 2004 Server 2004 - Consuming a Web Service
Microsoft BizTalk 2004 Server 2004 - Publishing a Web Service
Microsoft BizTalk 2004 Server 2004 - Integrating Business Rules
Microsoft BizTalk 2004 Server 2004 - Enabling Business Activity Services
Microsoft BizTalk 2004 Server 2004 - Enabling Business Activity Monitoring
Have fun!