You're right about them not being minified, but wrong about the need for 4 external stylesheets.

Look at their home page, for example. It uses just one stylesheet (v6core.css). That same file is also used on their blog, so my browser already has it cached. Perfect!

None of those stylesheets are less than 45K, so the savings they get from not needing to download them on every page outweighs the HTTP request hit they take by including them separately on more complex pages.