Summary of important user-visible changes for version 4.0:
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 ** For compatibility with Matlab, the "backtrace" warning option is now
    enabled by default.

 ** The preference

      do_braindead_shortcircuit_evaluation

    is now enabled by default.

 ** The preference

      allow_noninteger_range_as_index

    is now enabled by default and the warning ID

      Octave:noninteger-range-as-index

    is now set to "on" by default instead of "error" by default and "on"
    for --traditional.

 ** polyeig now returns a row vector of eigenvalues rather than a matrix
    with the eigenvalues on the diagonal.  This change was made for Matlab
    compatibility.

 ** Other new functions added in 4.0.0:

    validateattributes

 ** Deprecated functions.

    The following functions were deprecated in Octave 3.6 and have been
    removed from Octave 4.0.

      cut                polyderiv
      cor                shell_cmd
      corrcoef           studentize
      __error_text__     sylvester_matrix
      error_text

    The following functions have been deprecated in Octave 4.0 and will
    be removed from Octave 4.4 (or whatever version is the second major
    release after 4.0):

      allow_noninteger_range_as_index
      do_braindead_shortcircuit_evaluation

    The internal function atan2 of the sparse matrix class has been deprecated
    in Octave 4.0 and will be removed from Octave 4.4 (or whatever version is
    the second major release after 4.0).  Use the Fatan2 function with sparse
    inputs as a replacement.

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See NEWS.3 for old news.