Beta 4 is very likely to be the last test release before the full release of 0.9.9. The only thing left to do with it is to document and test migration path from WordPress installs.
There are two changes from beta3:
- Compatibility with the upcoming PHP 7.3 version.
- Fix for minor bug which in development enviroment caused JS errors as there was an attempt to use non existing unminified JS files.
You can download it for here, and read about the changes done in the 0.9 release line here.