Lots of work towards the v4 release in the past 18 days, among the changes since 4.0.6:
documentation has been moved out of the music21 directory into the root directory -- it is no longer installed with music21 from pip -- this change was necessitated by the move to retina quality graphics, but reduces the installation size from 90MB to 15MB for the full corpus version and 6MB for the no-corpus version.
But docs are still extremely important as witnessed by the 3 new user's guide chapters and removal of some obsolete docs. And all docs pass a new nbval test. nbval is now required to build docs.
Chord.add (was append()) and Chord.remove() allow for direct manipulation of bugs.
Improvements to parallel processing in music21.
Ottava spanners now come in two types, transposing and non-transposing -- reflecting whether the pitches under the spanner already reflect the transposition (non-transposing) or not.
Many improvements to spanners and RomanNumerals.
.exe files are no longer generated -- they were rarely used and pip is a better choice for Windows users now.