I just installed MT 4 and have tried various styles from the "Default Styles" selection, but when I try to change the layout (for example, with the "Hills Green" style, changing from "3-Columns, Thin, Wide, Thin" to "2-Columns, Wide, Thin", nothing happens -- the new layout is not applied. Am I missing something? Am I doing something wrong?

Ok. I was missing something...obvious. I didn't notice the note: "The selected theme has been applied, but as you have changed the layout, you will need to republish your blog to apply the new layout. Publish your site to see these changes take effect."

After republishing, the layout is applied. Duh. :-)