From the CC Blog:
The 4.0 licenses — more than two years in the making — are the most global, legally robust licenses produced by CC to date. We have incorporated dozens of improvements that make sharing and reusing CC-licensed materials easier and more dependable than ever before.
We had ambitious goals in mind when we embarked on the versioning process coming out of the 2011 CC Global Summit in Warsaw. The new licenses achieve all of these goals, and more. The 4.0 licenses are extremely well-suited for use by governments and publishers of public sector information and other data, especially for those in the European Union. This is due to the expansion in license scope, which now covers sui generis database rights that exist there and in a handful of other countries.
Among other exciting new features are improved readability and organization, common-sense attribution, and a new mechanism that allows those who violate the license inadvertently to regain their rights automatically if the violation is corrected in a timely manner.
Read the Complete Blog Post