The Halo components have been updated by default in Flex 4 to ship with the Spark skins. This means you can continue using the Halo components and still get the Spark skin. Also, if you want the Halo look in Flex 4, you can set the theme to Halo.
As per getting the Spark look in Flex 3, this isn't supported, but as I said, you can continue using the Halo components in Flex 4 and get the new look without migrating over to the Spark components. Is there a reason you'd like to stick with Flex 3?
"Is there a reason you'd like to stick with Flex 3?"
Yeah, another developer I work with had a big argument when I suggested we move to Flex 4 because it's not "production ready" or "production certified" whereas apparently Flex 3 is.