I want to do skinning like we use to do in flex 3 using flash designing some complex graphics. for example theres a button skin which has a gradient effect as well as a inner shadow on top and a white bevel in bottom. These typical skins are doable using flash where we can just refer the movie clip's instance name in flex 3 css file. But how to do the same in flex4.
Actually skinning is by far easier and better in Flex 4, you don't need to depend on external files.
You can have a full button with the looks that you want quite easily.
Read up some on Flex4 skinning. There are tons of examples out there.
You can also use Catalyst and create your assets in Illustrator and generate Flex 4 skins, which you can easily modify later inside FlashBuilder if you feel like it.