Latest update 22/5/14: It's early days, but the panda 4.0 update has rolled out in the UK, and preliminary findings are that sites with "thin content" (that is sites with very little content or content duplicated from elsewhere) have been impacted.
We are running analysis on our own clients sites at the moment, and will report our overall findings here (clients will be contacted individually with their own results.)
Breaking news 21/05/14: Matt Cutts, Googles Head of Webspam, has announced on twitter that Google Panda 4.0 is currently rolling out. While there have been several minor refreshes over the past year, the fact that this one has been officially announced makes us think that it is a full on algorithm change.
Barry Schwartz at Search Engine Land spoke with Google and believes this could be a big release:
"Google told us that Panda 4.0 affects different languages to different degrees. In English for example, the impact is ~7.5% of queries that are affected to a degree that a regular user might notice."