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 and finish/

While importing choose the default SDK (Flex 4), and check the Use Flex 3 Compatible mode.