No, not really. I have both Flexbuilder 3 and Flex builder 4 myself.
However, the project has be accessible by both Flexbuilder and especially SDK users and should compile against the latest Flex compiler which is 4.5 otherwise it will become dead or abandoned again next year.
The latest microphone echo cancellation feature in Flex 4.5 is a very compelling reason to make sure all Flex projects are 4.5+ compatible. I use XIFF in my other projects and it is already 4.5 compatible.
I have now managed to get to compile with 4.5 