Today KDE released updates for its Workspaces, Applications, and Development Platform.
Significant bugfixes include improvements to the Kontact Suite, bugfixes in Dolphin and many more corrections and performance improvements all over the place. The changelog and Bugzilla list more, but not all improvements since 4.8.3.
These changelogs are incomplete. For a complete list of changes that went into 4.8.4, you can browse the Subversion and Git logs. 4.8.4 also ships a more complete set of translations for many of the 55+ supported languages.
To download source code or packages to install go to the 4.8.4 Info Page.