I have started looking into Flex 4 (Gumbo) and below are a couple of links that I have stumbled across with more information about Flex 3 & Flex 4. As I find more links I will try and keep this list updated. Please add any additional links through the comments or send me an email and I will make sure.
Links:
Differences Between Flex 3 and Flex 4 beta: Written by Joan Lafferty, this article focuses more on low level styles and changes and is well suited for someone looking into the visual aspects of the new platform. Overall I felt that the content was excellent and a nice walk through on some of the key differences in the two releases.
What's New In Flex 4: Matt Chotin gives a very high level overview on some of the new visual features in flex 4. The article is more for a 10,000 foot overview on the key design changes, but offers some good links to other more technical articles.
What's New in Flash Builder 4 Beta: I have to admit that I am more of a data guy then a graphics person and so far most of the changes where about the display side of things. This article highlights a new "Data Centric Development" method that focuses more on the data side of things. Tim Buntel gives a high level and intriguing look at some of the data enhancements to the platform. I am sure this will be a topic for more posts.