Dreamweaver CS4 does not support HTML5 tags however, you may add this functionality yourself fairly easily.
To do this go to Edit > Tag Libraries
Click the + button on the top right and select New Tag.
The select the tag library: HTML Tags and enter the tag you wish to add there.
Tip: You may enter multiple tags at once by separating them with commas like this:
article, aside, audio, canvas, command, datagrid, datalist, details, dialog, figure, footer, header, hgroup, mark, meter, nav, output, progress, rp, rt, section, source, time, video