Set version to 0.4.0 for release.
Reverted version to 0.4.x after 0.4.0 release.
Merge remote-tracking branch 'origin/v0.4.x'

* origin/v0.4.x:
  Reverted version to 0.4.x after 0.4.0 release.
  Set version to 0.4.0 for release.
Changed development version to 0.5.x.