Thank you very much Cliff.
I tried this and I found common problems on with the migration of an application from Flex 3 to Flex 4.
To solve some of them, look at :
http://www.lynchconsulting.com.au/blog/index.cfm/2010/3/21/Flex-3-to-Flex-4-Migration-HowtoI decide to go back on Flex 3 because Flex 4 is to young.
Regards
Willy