FOF and FEF are not used in any of our native Joomla!™ 4 extensions; we use the native Joomla 4 MVC and the Bootstrap framework shipped with Joomla!. So, don't worry about removing FOF 4. It's something you have to do manually anyway.
Using MagicEraser is only validated on Joomla! 3. I don't know what will happen on Joomla! 4. Take a backup, try to run it, and then install Akeeba Backup Professional on your site, twice in a row, without uninstalling it before or in between. This will ensure that all of the correct files are present.
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!