Default WordPress menu content

by

Use the fallback_cb argument of wp_nav_menu():

(callable|bool) If the menu doesn’t exists, a callback function will fire. Default is ‘wp_page_menu’. Set to false for no fallback.

<?php
wp_nav_menu( array( 
    'theme_location' => 'main_menu_4',
    'fallback_cb'    => false
    'depth'          => 1, 
) );
?>