Authenticated users can't view the front page
Building on Patrick Kenny's comment, I'd recommend looking for the 'URL redirects' page within your admin pages. (You will have this page if you've installed a module which supports URL redirects - 'Redirect' in D7, 'Path redirect' in D6.) Then see if any rule is defined there which'd redirect visitors from site.com/node/4 to site.com/user/4 .

If this module isn't doing this, odds are another one is, so as Patrick says start disabling contrib modules one by one.