Whether you respond as a 404 or a 403 would be dependent on whether or not the user who is logged in has the authorization to read the previous directory.

A site administrator, for example, would have the authority to read the previous directory, which means that the site administrator would know whether or not the resource existed or not(as the previous directory would list it) so in which case a 404 would be proper. However, a user who doesn't have authority to read the previous directory should not have the ability to know whether or not it exists. so a 404 would not be proper here because the proper one would be a 403 because it's inherited from the previous directory.

edit: changed traverse to read, as traversal doesn't mean you can see what else is there.