I just downloaded 1.4.4 and am having the same problem. I can see the category-sitemap.xml and post_tag-sitemap.xml, but not the post-sitemap.xml or page-sitemap.xml. When I click on the link, I get the message from my 404 template.
Doesn't seem like it should be a permissions issue, since 2 out of 4 are being found. When I add a new post or page, I see that these xml files are updated on the fly. But I still get the error after the file is regenerated.
Appreciate any insights.