Drupal 7 and Views 3: Page Display and Random Appended Arguments in URL
5898 is just an extra URL argument in View's point of view. Views doesn't have the ability to throw 404 pages because there are "extra" arguments in the URL. Views does throw 404s when there is no argument in the URL (if configured that way). In any case, you can implement something like hook_views_pre_view() and redirect the user to a separate 404 page if you see that extra arguments are found in the URL. This should take like 15 minutes for a Drupal developer to do.