Sonos has leaked that it will be announcing a new subwoofer soon. The new subwoofer is not a Gen 4 model though, which would be a successor to the current Sonos Sub.
https://www.notebookcheck.net/Sonos-Sub-Mini-Compact-subwoofer-turns-up-in-the-Sonos-app-ahead-of-its-unveiling.582224.0.html