Update from 3.9.x to 4.x

Just an FYI that Joomla 3.10-alpha has now been released so the upgrade process can now easily done.

Instructions:

1. Go to the Joomla Update Component Options and switch the Update Channel to Testing and the Minimum Stability option to Alpha.
2. Update to 3.10 alpha1 using the Joomla Update Component.
3. After you have updated to 3.10 alpha please switch the Update Channel to Custom URL and set the Custom URL to https://update.joomla.org/core/test/310to4_list.xml
4. After the upgrade please switch back to the Testing Update Channel with the Beta minimum stability to get future updates between Joomla 4 betas.

You need to wait for 3.10.X to become available; as per the documentation, "This guide assumes you are starting on Joomla 3.10.x. If you are on a earlier version make sure you upgrade to Joomla 3.10 first before moving to Joomla 4."

Joomla 3.10 is currently in development, at 86% completion, most notably, with no completion date for release. V3.10-dev can be found here but is not yet available for updating via the Update process in your current Joomla 3.X installation.