How to remove dropdown items from a form?
If the name of your field is edit-taxonomy-forums-und your code should be:

unset($form['edit-taxonomy-forums-und'][LANGUAGE_NONE]['#options']['4']);


Maybe you don't have the correct name of the field. Go to the Manage fields page of your content type (YOUR_SITE/admin/structure/types/manage/YOUR_CONTENT_TYPE/fields) and check the name of the field.



EDIT:

If you read the comments in the answer you will see that the problem was the change of forum_node_form by forum-node-form.