The plan:

April 17, 2014 Trunk is open for business.
April 30, 2014 (+2w) Initial 4.0 meeting.
May 21, 2014 (+3w) Decision time for features being developed as a plugin. Merge window opens for any blessed for 4.0.
June 4, 2014 (+2w) Feature plugin merge window closes.
July 9, 2014 (+5w)
July 10, 2014
Beta 1.
From this point on, no more commits for any new enhancements or feature requests in this release cycle, only bug fixes and inline documentation. Work can continue on enhancements/feature requests not completed and committed by this point, and can be picked up for commit again in just a few weeks at the start of WordPress 4.1.
July 16, 2014 (+1w)
July 18, 2014
Beta 2.
July 23, 2014 (+1w)
August 6, 2014
Beta 3.
July 30, 2014 (+1w)
August 15, 2014
Beta 4.
August 6, 2014 (+1w)
August 27, 2014
Release candidate. (More release candidates as necessary.)
Week of August 25 (+4w)
September 4
Target date for release of WordPress 4.0.

To get involved in WordPress core development, head on over to Trac and pick a 4.0 ticket. Need help? Check out the Core Contributor Handbook. Get your patches done and submitted as soon as possible, then drum up people to test the patches and leave feedback on the ticket. Patches for enhancements won’t be committed after the posted dates, so that we can all focus on squashing bugs and hopefully deliver the most bug-free WordPress to date. Wish us luck!