PSR-4 was voted in as an "accepted" PSR by the FIG in December. It took a little while to get done and went through a series of painful rewrites but when we have in the end is a document that reflects what this truly is: an improvement on PSR-0.
Today Jordi Boggiano merged a pull request by Andreas Hennings into master branch of Composer that contained support for PSR-4. Andreas was a massive help to the FIG while we were trying to shake the issues out of PSR-4 during Draft and Review stages, so he really outdone himself by providing the code too.
Phil makes a few suggestions about moving to PSR-4 including: not moving immediately, making a "psr4" branch to test it out and points to an example of how to do it. More information on PSR-4 and Composer can be found in the official documentation.