Migrating from Flex 3 to Flex 4

One of the simple trick and very important one is to add following line in Flex compiler. Right click on Flex project name from Flex 4 & select "Properties" option (last one). In open window click on "Flex Compiler" and following line at the end.

-compatibility-version=3.0

After that compile your application and you can see Flex 3 apps migrated or compiled in Flex 4.

If you have CSS issues at this line in your Flex code.



@namespace "http://www.adobe.com/2006/mxml";



PS: Adobe is going to release Flex 4 sometime end of March 2010 or April 2010, so it almost about the time you should to start thinking of converting your existing Flex 3 apps into Flex 4.

Flex 4 has simplified State declaration and make it very easy to implement. I think it is worth the efforts to convert even if your existing one works fine.
Cheers