If you post a new review, and go to the review page, but you are not logged in, the only link that is displayed is "download diff".  Clicking this gives you a 404 error.  It seems like the best thing to do is to use the same rules for showing the "view diff" link.

Looked at new, unpublished review while logged out, saw no links.
Looked at new, unpublished review while logged in, saw 5 links.  Download diff/view diff work as expected.
Looked at a review with an unpublished update.  When logged out, saw 2 links (download diff/view diff which show you the previous published diff).  When logged in, saw 5 links.