To import a Flex Builder 3 project file in Flash Builder 4 follow the steps below:
1. Choose File -> Import -> Other
2. General -> Existing Projects into Workspace; and click next
3. Select the root directory or archive file; and click browse. Locate the previous Flex Builder 3 project.
4. Finish
While importing Choose the default SDK (Flex 4), and check the Use Flex 3 Compatible mode.