What OS are you using?
In Vista go to ControlPanel/Default Programs/Associate a File Type or Protocal with a Specific Program
scroll down to and select .fla file extension and set the association (Change Program) to CS4.
Similar in most systems, file type associations are the keywords.
Right click on file/Get Info/Open With/Change All
Again, if I choose new in CS4 choose the AS, then save , making sure it is saving as a CS4 document. Save close CS4 go to open the CS4 project I just saved it is a CS3 icon. Doubleclick the icon and the doc opens in CS4. Of course I tried open with and then choose CS4 but that means after I create projects in CS4 I have to do this to every project? Not what I had in mind to do as projects saved in CS4 should have CS4 icons and open on a doubleclick with CS4, why is the saved CS4 doc defaulting to a CS3 icon?
I think I know what the problem is. I have the CS3 master suite apps on my main drive and a copy of the CS4 master suite on that drive as well, all except Flash CS4 which I have on a second drive.
In Photoshop, Illustrator or any other CS4 app the default icon is CS4 even if I open a doc in CS3 so with Flash CS3 on ,my main drive and CS4 on the second drive this would explain why Flash saves as a CS3 icon rather than CS4. only way to see is uninstall Flash CS4 from the second drive and add to the main. I'm a bit worried to do this as I use Flash for a living and hope I do not mess the preferences or have a problem locating previous files.
On second thought maybe I should just install Flash CS4 on my main drive with the rest of the CS4 master and boot from that.