If you wanna be part of public beta program, then enroll yourself for the Apple Beta software testing program for OS X for free. There is no detailed release note information about the new Public Beta build of Mac OS X 10.11 El Capitan.
How to get Mac OS X 10.11 EL Capitan Public Beta 4
If you are running the previous version of public beta i.e., El Capitan Public Beta 3, then you can update to latest build of beta 4 by going to Mac App Store and then hit the Updates tab. It will then list down available existing updates and definitely it will be Public Beta 4 available there. Click on Update button and then click on “Download and Restart” button.
The update can take couple of minutes, depending upon your Internet Connection and then reboot your Mac to get the latest update settings.