I would think that Dreamweaver 8 is the only version that Webstyle 4 will work within. Webstyle 4 was released in 2004 and has not been updated since that time. Xara retired the program almost 2 years ago.
Rather than using the Dreamweaver addin you could use Webstyle 4 as a standalone application then manually add the code to include the Webstyle output to your Dreamweaver project.
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