Confluence 4's Import from Disk feature uses the undocumented XML format created by the Export to XML feature.
I experimented with using Webworks ePublisher Designer to convert from FrameMaker to Confluence 4. It looks fine, but when you edit a page the code is an uneditable mess of macros and whatnot due to Webworks using Confluence 3.x wiki markup rather than Confluence 4 XHTML. It would be fine for read-only publishing.
