Authenticated users can't view the front page I set node/4 as the front page on the Site Information screen.

This works for anonymous users, but authenticated users cannot access the front page (node/4).  Even if they manually type www.site.com, they are redirected to www.site.com/user/4.

Please help me out.
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.