Maintained projects that rely on Joomla will be updated on Joomla 3.10 per the schedule and eventually updated to Joomla 4 probably by the end of 2022.
This will be the last beta of Joomla 4 and the Release Candidate will be available in March. A new portal is also available for translation teams to submit Joomla 4 language packs to coincide with the first Release Candidate.
If you want to download, install and test the package to improve quality assurance for the forthcoming 4.0 release and to discover the new features introduced in this major release.
Try Joomla 4.0 Beta without migrating one of your sites? Test it directly at launch.joomla.org
Joomla 3.10 helps with the transition to Joomla 4
Along with Joomla 4 Beta 7, there will be a release of the fifth alpha version of Joomla 3.10.
Joomla 3.10 is the bridge between the 2 major series. Joomla 3.10 stable will primarily be a release containing backports of API changes from the Joomla 4.0 development branch to help ease the transition to the next major release for the community. Joomla 3.10 will be supported for 2 years after it has been finally released.
Joomla 3.10 will introduce a new feature to the Joomla Update component to help with the mini-migration process: the Pre-Update Checker.
Once a website is updated to 3.10, the pre-update checker will allow the compatibility check with the Joomla 4.0 minimum requirements in terms of the current hosting environment and, more importantly, your installed extensions. This will help with migrations to Joomla 4.0.
Find out more read documentation about Joomla 4
There are some tutorials and there will be more to help us understand the significant changes in Joomla 4. You can find the existing ones, like how to create a Plugin or a Module for Joomla 4, namespaces conventions, prepared statements, using the new web asset classes, and many more here.
You can check documentation regularly, update it and provide the missing content at docs.joomla.org