Audacity 4.0 rebuilds the interface on Qt, adds the one-way .aup4 project format, and ships without the scripting pipe

Sources: release notes, discussion
Summary: The release notes state the interface is rebuilt on Qt, and that .aup3 projects convert to the new .aup4 format and cannot be saved back. They list Macro Manager, the scripting pipe, MIDI tracks, the mixer, and VAMP and LADSPA plugin hosting as absent from 4.0. No replacement for the removed automation surface is stated.
Why it matters: Anyone driving Audacity from a script has no upgrade path in this release, and the project format conversion is one way.
Follow-up: Track whether the scripting pipe and Macro Manager return in a later 4.x release.