I'm assuming you are referring to an FLA file that what made in Flash CS4 (version 10) that you'd like to open and edit in Flash MX 2004? If this is the case, you need a bunch of software
Flash CS4 files can be saved back to Flash CS3 by choose File > Save As and choosing Flash CS3 Document from the "Save as type". Then you need to open the CS3 document with Flash CS3 and do the same thing (back to version 8). Repeat the process one more time with Flash 8 and you'll be back to Flash MX 2004 (version 7).
Now if you are simply trying to export an SWF that is compatible with Flash Player 7, you can do this directly from Flash CS4 (File>Publish Settings>Flash). The only tricky part is if you are using ActionScript 3 (or any other features like Filters, 3D, Components, etc). You need to convert the code to ActionScript 2 (and there's not tool for that). In this case, it might be easer to start from scratch. Make a new document. Go to your directly to your publish settings (mentioned above) and set your Flash Version to 7. Now Flash won't let you use anything not compatible with version 7.