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Make sure to do a database backup before you upgrade since version 4.x is not backwards compatible with version 3.x. Hermod version 4.x contains significant changes in the database schemas and hence automatic downgrade to version 3 or lower versions is not supported. Upon launching Hermod 4.x it will automatically update the database configuration to the new structure.
Save the configuration file and restart Hermod.
Test your new configuration on a non-production server, to make sure it works as expected.