If you want to upgrade a project in version 4.x to the new version you will probably need to refactor all your process to the new process language BPMN 2. The APIs and the services has been changes as well. I strongly recommend you to take a look at the new documentation to understand the new components and concepts. As I mention here : http://community.jboss.org/message/621560#621560 I'm writing an updated version of the book jBPM Developer Guide to this new version.
Cheers