COMSOL Multiphysics version 4.0 is in many aspects an entirely new product. The release notes outline the differences between version 3.5a and version 4.0. It also outlines additions that we intend to make in COMSOL Multiphysics version 4.0a.
We have strived to achieve backward compatibility with the previous version and to include all functionality that is available there. However, version 4.0 is not fully backward compatible with version 3.5a.
All backward compatibility issues are planned to be solved for version 4.0a unless explicitly stated.
In the menu to the right you will find release notes for each product.