There may be various approaches to use Flash Builder 4 with your existing Flex 3 projects. The approaches can vary depending on the Flex SDK features you need and the kind of migration that you are ready to undergo.
The transition from Flex 3 apps (developed using Flex Builder 3) to Flash Builder 4 does not require migration of the app to use Flex 4 SDK. This gives you the liberty to avail the benefits of the features of Flash Builder 4 right away on the existing project.
Some features of Flash Builder 4 features comprise a network monitor, enhanced debugging, refactoring support, code editing improvements, unit testing support, data-centric development (DCD) workflow and more.
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Tags: Adobe, Flash Builder 4, Flex, web development