Habs geschafft.

SELECT t1.threadid ,t1.threadtitel,t1.threadtype,t1.postcounter,COUNT(t4.postid) AS numPost  
FROM mainproject.board_thread AS t1  
  
LEFT JOIN (SELECT DISTINCT ON (t3.tid) t3.tid,t3.thetime  
			 	FROM mainproject.users_follow_disc AS t3  
			 	WHERE t3.uid=".$_SESSION['userid']."  
				GROUP BY t3.thetime,t3.tid  
			 	ORDER BY t3.tid,t3.thetime) AS thepost  
ON thepost.tid=t1.threadid  
  
LEFT JOIN mainproject.board_category AS t2 ON categoryname='".$tag."'  
	  
LEFT JOIN mainproject.board_post AS t4 ON t4.threadid=t1.threadid AND t4.creationtime > thepost.thetime  
  
WHERE t1.categoryid=t2.categoryid  
GROUP BY t1.threadid,t1.threadtitel,t1.threadtype,t1.postcounter  
ORDER BY postcounter

Und einen Fehler in der DB behoben das immer geupdatet wird wenn derjenige im thread war.