Whats different in version 4
As mentioned above, V4 is a complete refactor. Much of the actual code is the same but has been modernized for J4 with the intent on running on J5.
There are some operational differences and things you need to be aware of.
Attachments are now stored in an attachment folder rather than the Database. You can configure where you want these attachments stored via the component options configuration.
We no longer run the delivery or spamcheck scripts natively. Instead we use the Joomla CLI interface.
V4 cannot use pre-V4 database tables without them being upgraded. This upgrade is performed during installation. Once upgraded you cannot go back. See details in the upgrading section.