The only reason why we have SWC for different versions of the Flash player is because we can't support new feature and keep backward compatibility (i.e. 10.x added support for rtmfp and p2p, and 10.3 added support for echo cancellation) but otherwise you can use any of them with any higher version of the Flash player (I tend to use the Flash 9 version for everything unless I need some of the newer features).
If you are talking about a specific version for Flex 4 (i.e. Spark components) we have an alpha/beta version posted on the forum but development has been stopped so if you want to use it you'll have to take it from there.
