Backwards Compatibility for Source-Connect 4
Source-Connect 4.0 has been completely redesigned to improve both the user interface and the application’s performance. Features such as your contacts, dashboard profile, subscriptions, and support contracts are completely compatible with older versions of Source-Connect (such as 3.9 and 3.8).
Having said that, advanced features such as Restore/Replace, the Source-Connect Link plugin, and the Remote Transport Sync (RTS) feature are not backwards compatible. You will still be able to connect to your contacts, but these functionalities will only work between 4.x or 3.x versions.