Use SWF/SWC compiled in Flex 3 within Flex 4 application

Technology named Marshall Plan can separate contexts the applications are run in. A Flex 3 library or application should be embedded into Flex 4 application in so-called 'sibling context' not to interfere with Flex 4 namespaces and classes. If there's no option to re-compile Flex 3 code with Flex 4 SDK, then simply load Flex 3 application with code like this:

<mx:SWFLoader id="legacy" source="legacy.swf" loadForCompatibility="true" />

Note last clause - this tell Flex 4 to use Marshall Plan and load SWF into sibling context, not into main context (otherwise it will conflict).

If you decide to move part of business logic into Flex 3 SWF and use within Flex 4 application via Marshall Plan, you cannot just access SWF's data. You'll have to use events and listeners to communicate between the two.

Sending event from Flex 3 SWF:

var e:Event = new Event("FOOEVENT");
this.systemManager.loaderInfo.sharedEvents.dispatchEvent(e);

Listening for event in Flex 4 app:

contentLoader.content.loaderInfo.sharedEvents.addEventListener(
'FOOEVENT' , onFoo );

private function onFoo(evt: Event): void {
            // do something  
}

Marshall Plan can also be useful if loading SWFs into application from different security contexts (remote vs. local).