Ruby on Rails 4.0 beta 1 was released recently on 26th February 2012.
Want to explore new features in rails 4, Upgrade your rails to 4.0 beta 1 as
gem install rails --version 4.0.0.beta1 --no-ri --no-rdoc
Disclaimer : Rails 4 clearly breaks a lot of stuff. Read the changes before upgrading your application.