Several data files created and maintained by Source Insight have changed in version 4.
Project Files - the new extension is .siproj for the main project file. All the project data file formats have changed from version 3. You can still open version 3 projects, but they will be converted to the new format.
Configuration Files - all configuration settings are now saved in XML files. Because the configuration changed so much, version 3.5 configuration files are not supported.
Workspace Files - new extension is .siwork. These are not compatible with version 3.
Recovery Files - the new extension is .si_recovery.