Joomla 4 RC3 download is available for testing. JoomlArt team have researched and got you some FAQs for your concerns about this new upgrades. We will keep posting new questions and answers frequently.
Joomla 3 and Joomla 4 features comparison: 15 new things to take note
Joomla 4 alpha 1 was released on 17 November 2017 with huge updates and major developments to improve user experience (UI), code quality and giving Joomla! large functionality improvements with limited backward compatibility breaks. Let’s do feature comparison between Joomla 4 and Joomla 3.