It's not possible.
The only way that comes close is to copy and paste individual items from one open instance of CS5 to an open instance of CS4. Or you could maybe save as a Premiere Pro project and import that into CS4. Either way is tedious and loses a lot of information.
If you want this ability, you can file a feature request here:
http://www.adobe.com/go/wish
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Todd Kopriva, Adobe Systems Incorporated
Technical Support for professional video software
After Effects Help & Support
Premiere Pro Help & Support
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