I have explained that in https://www.akeeba.com/news/1747-joomla-4-is-around-the-corner.html
Update our extensions to the latest version. Do note that the latest versions for Joomla 3 are also compatible with Joomla 4, as has been the case since November 2017.
Follow the instructions under “Before upgrading an existing site to Joomla 4” first.
Upgrade to Jomla 4.
Upgrade to the native Joomla 4 versions described at the top of the article. Akeeba Backup 9 is the only extension you will need to install manually; the other ones will appear as updates about a day after you upgrade to Joomla 4.
Nicholas K. Dionysopoulos
Lead Developer and Director
🇬🇷Greek: native 🇬🇧English: excellent 🇫🇷French: basic • 🕐 My time zone is Europe / Athens
Please keep in mind my timezone and cultural differences when reading my replies. Thank you!