Starting with version 4.0, the add-on is now an Atlassian paid add-on - see this announcement.
Once 4.0 or 4.1 release is installed, Confluence will automatically start migrating Confluence 3.x style macros supported by this add-on to Confluence 4.x native macros. For this reason, it is not easy to move back to a previous add-on version.
Product levels were not changed with this release to enable upgrades to Confluence 4.x macros without other changes. Another version with updated product levels will be released in the near future. Open issues if you have specific requirements.
All Full/Production downloads for this product are available through the Atlassian Marketplace. In addition, Early Access Program (EAP) downloads are also periodically available here for use at your own discretion.