I'm an Administrator user running a fresh install of today's 4.0 beta 1 build. I installed and activated the WordPress Importer plugin. I ran the Importer, selected my XML file, and clicked Import.
On the next screen, the Importer does not run - I see the screen heading but nothing else. The XML file I attempted to import appears in the Media Library:
(I have trouble wrapping my head around Trac. Tried finding a ticket related to this, but alas...!)