In previous articles Flex SDK 4.0.0.14159 Omit Trace Bug And Solutions or Flash Builder 4 Compiler Bug ( FB 4.0 build 272416 ) And Solutions, we hacked some Flex SDK 4.0 bugs(although unrelated, ).  June 30, 2010, The official Flex team announced the release of Flex SDK 4.1 and Flash Builder 4.0.1 which are available for download.

The update include:

• Support for mirroring layouts and text in order to support right-to-left- locales. The goal of this new feature is to trivialize the work involved to re-purpose Flex applications designed for left-to-right locales for deployment in right-to-left locales.
• Native support for Flash Player 10.1 and AIR 2.
• Critical bugfixes and enhancement requests.

The Flex compiler is way behind Java in terms of compile times, since Flex 4 adobe made compiler performance improvements, Andrew Shorten, Senior Product Manager at Adobe, said Adobe would continues to be a goal of each release of Flex(details here). It seems not so much to be done in this release.

In addition, Adobe Flash Builder 4 software is designed to help software developers rapidly develop cross-platform rich Internet applications (RIAs) and content using the open source Flex framework. It includes support for intelligent coding, debugging, and visual design and features powerful testing tools that speed up development and lead to higher performing applications. 

With the move from Flex 3 to Flex 4, there seemed to be some changes to the overall Flash platform, including a renaming of Flex Builder to Flash Builder.

More details about this upadate:

Flex SDK 4 and Flash Builder 4 Updates Now Available!