Randomizing your tags doesn’t matter much to WP_Query. If you want random results, try adding 'orderby' => 'rand' to your $args. This should randomize the stories returned. If not, try applying shuffle() to $my_query before iterating through them. Some web hosts have reportedly disabled the 'orderby'=>'rand' method of querying posts. For these hosts shuffle() seems to still get the desired results.
NOTE: If you go with the shuffle() method, you probably want to replace the posts_per_page value with ‘-1’ and add a loop counter to your output loop. Otherwise, you will always be shuffling the first four stories.