If the CS4 swf calls Font.registerFont, it should work.
You may also want to look at the various embedded font format issues. Many of the Flex 4 components use a newer embedded font fortmat, which is not the same as the one generally used in Flash. So even if ther Flash font gets registered properly, it still may not work with some of the spark components.
You should be able to create a swf using Flex that contains fonts embedded in either format, which can then be used in the way you describe, though, since Flex 4 suports both font formats.
Hopefully, someone else will jump in here if I got some of this wrong.