Not for 'how-to' coding questions but PHP theory instead, this forum is here for those of us who wish to learn about design aspects of programming with PHP.
$result4 = mysql_query("SELECT ua.user_username AS author_username, ua.user_username_base AS author_username_base, ua_last.user_username AS last_username, ua_last.user_username_base AS last_username_base, ft.forum_thread_id, ft.forum_thread_count_posts, ft.forum_thread_count_replies, ft.forum_thread_count_views, ft.forum_thread_name FROM forum_threads AS ft LEFT JOIN user_accounts AS ua ON ft.forum_thread_author_id = ua.user_id LEFT JOIN user_accounts AS ua_last ON ft.forum_thread_author_id_last = ua_last.user_id WHERE forum_thread_forum_id='".$forum_id."' LIMIT 0 , 30");
$row4 = mysql_fetch_array($result4);
echo '<pre>';
var_dump($row4);
echo '</pre>';