I couldn't figure out any obvious way to manually add Top Sites in Safari 4. I tried drag-and-drop, but it wouldn't work. After some digging, I found a TopSites.plist that lets me add sites manually. I posted a how-to on my blog:
http://micronetsoftware.com/blog/2009/02/26/adding-top-sites-in-safari-4-beta/
http://micronetsoftware.com/blog/2009/02/26/adding-top-sites-in-safari-4-beta/