RAD Studio 10.4 has a nice set of new features, now covered in several blog post and in the announcement webinar (the replay is available at https://www.youtube.com/watch?v=-rTLEXLVueQ).
The 10.4 release also addressed a large number of issues, including fixes for over 700 publicly reported issues on Quality Portal (QP). You can see a (partial) list of the fixed issues in the product documentation at: http://docwiki.embarcadero.com/RADStudio/Sydney/en/New_features_and_customer_reported_issues_fixed_in_RAD_Studio_10.4
The large number of issues resolved is also visible in the Created vs. Resolved chart (limited to bugs) on QP:
This is a continuously updated graph that you can see in one of the QP dashboards at https://quality.embarcadero.com/secure/Dashboard.jspa?selectPageId=13100
Finally, this is a breakdown of portion of the bug reports and new feature requests addressed in 10.4. The items are grouped by product area, with some of the categories overlapping (for example Compiler and Tools includes C++ and Delphi compiler issues, but also Linker issues) and issues possibly listed in more than one category: