Almost a year ago Jon Rose on InfoQ published an article “Top 10 Changes in Flex 4”. This article is still actual for you if you intend to examine new Flex 4, released not so late.
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Top 10 Changes in Flex 4
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Flex SDK 3.5A released
Flex SDK 3.5A (3.5.0.12683) was released a week ago. It contains quite a few changes comparing to its previous release. You can download it at official Adobe site.