tl;dr: Audacity 4.0 rebuilds the UI on Qt with high-DPI rendering, dockable panels, workspaces, themes, and a redesigned clip-editing model that allows multi-clip selection, grouping, free placement, and context-sensitive tools replacing the old Select/Envelope/Draw modes. It introduces a new .aup4 project format (backward-compatible with .aup3 imports), ASIO support on Windows, revamped playback/recording, and consistent effect handling. Several Audacity 3 features—Time Tracks, MIDI tracks, Mixer, Macros/scripting, VAMP/LADSPA hosting, and Play-at-speed—are not yet available but planned for future releases.