The first release candidate of rails 4 is out!
Also security issues are considered. Now the session store is encrypted by default, there is better assignment-attack prevention.
The final release should be released in about 3 to 4 weeks. An upgrade guide how to convert from rails 3.2 to rails 4 is available.
You can install the release candidate without the documentation with:
gem install rails --version 4.0.0.rc1 --no-ri --no-rdoc