Release 4.0 adds support for 3rd-party packages to provide PyInstaller hooks along with the package. This allows Maintainers of other Python packages to deliver up-to-date PyInstaller hooks as part of their package. See our sample project for more information.
PyInstaller uses this option itself to provide updated hooks much faster: Many hooks are moved into the new package pyinstaller-hooks-contrib which is updated monthly. This package is installed automatically when installing PyInstaller, but can also be updated independently.
Finally, this version drops support for Python 2.7, which is end-of-life since January 2020.. The minimum required version is now Python 3.5. The last version supporting Python 2.7 was PyInstaller 3.6.
You can find a detailed list of changes in this release in the change log section of the manual.