Version 4.x has been specifically redesigned with usability in mind. Now you can be more productive than ever with Poseidon, using particularly the following enhancements:
UML 2.0 Activity Diagrams with intuitive user interface
Improved drag and drop in sequence diagrams (now includes execution occurences and asynchronous messages)
Improved UMLdoc
More configuration options
Support for new UML 2.0 Activity Diagrams
Roundtrip bug fixes
A complete list of changes can be found at the change log section of the Gentleware web site.