Matt Cutts tweeted confirmation of Google rolling out Panda 4.0 update. US based at present.
Google is rolling out our Panda 4.0 update starting today.
— Matt Cutts (@mattcutts) May 20, 2014
The Panda algorithm was released in February 2011 to target sites with low quality content and has been updated over time.
You can view the change history of Panda thanks to Moz.
What 4.0 will bring will be interesting to see. If you’re seeing any changes then add your comments for discussion.