What I mean is to say is can you create components for flex 4 sdk using component kit?
The latest Flex Component Kit for Flash (v2.0.0) should work fine with Flash CS4 and Flex 4. It does not support Flash CS3.
Here is a link to the spec for the changes in Flex 4:
You should be able to download version 2.0.0 of the MXP in the open source repository: