Andrew Nacin. writes “An RC comes after the beta period and before final release. We think we’re done, but with millions of users and thousands of plugins and themes, it’s possible we’ve missed something. So if you haven’t tested WordPress 3.4 yet, now is the time! Please though, not on your live site unless you’re adventurous.
With more than 500 tickets closed, there are quite a few changes. Plugin and theme authors, please test your plugins and themes now, so that if there is a compatibility issue, we can figure it out before the final release.
If you are testing the release candidate and think you’ve found a bug, please post to the Alpha/Beta area in the support forums. If any known issues crop up”
Read full post at: http://wordpress.org/news/2012/05/wordpress-3-4-release-candidate/