Check out Joan's whitepaper on Flex 3 to 4 migration: http://www.adobe.com/devnet/flex/articles/flex3and4_differences.html
In my cursory understanding of this issue, there are 2 ways to do what you're trying to do:
1. (Preferred, slightly more overhead) Switch your project to a Flex 4 project.
You can still use the exact same Flex 3 code, but compile it with the Flex 4 SDK.
Probably the easiest way to do this is to open your project in Flash Builder 4 Beta, then change the SDK from project properties > Flex Compiler
You may want to use the -compatibility-version=3 compiler flag. This page (http://labs.adobe.com/wiki/index.php/Flex_4:Backward_Compatibility) describes what that flag does.
Embed your Catalyst-created SWF using SWFLoader or Image, in the normal fashion. You should set loadForCompatibility="true"
2. (Easier, but not really supported) Keep your project as a Flex 3 project, but update some properties.
In project properties > Flex Compiler, make sure you are using at least SDK 3.2 - this is the first version that supports loading Flex 4 content inside a Flex 3 app.
In project properties > Flex Compiler, change the "Require Flash Player version" to 10.0.0.
Embed your Catalyst-created SWF using SWFLoader or Image, in the normal fashion. You should set loadForCompatibility="true".
Let me know if you need more details. You can probably get a more complete response on the Flex 4 beta forums ( http://forums.adobe.com/community/opensource/flexsdk/general ).