SELECT * FROM ( 
  SELECT id, 1 as post_type, last_comment_time FROM posts WHERE owner= 3494430 
  UNION 
  SELECT id, 2 as post_type, last_comment_time FROM photos WHERE owner= 3494430 
  UNION 
  SELECT id, 3 as post_type, last_comment_time FROM photoposts WHERE owner= 3494430 
  UNION 
  SELECT id, 4 as post_type, last_comment_time FROM discussions WHERE owner= 3494430 
  UNION 
  SELECT id, 1 as post_type, last_comment_time FROM posts WHERE id IN ( 
    SELECT post FROM subscriptions WHERE user = 3494430 AND post_type=1 ) 
  UNION 
  SELECT id, 2 as post_type, last_comment_time FROM photos WHERE id IN ( 
    SELECT post FROM subscriptions WHERE user = 3494430 AND post_type=2 ) 
  UNION 
  SELECT id, 3 as post_type, last_comment_time FROM photoposts WHERE id IN ( 
    SELECT post FROM subscriptions WHERE user = 3494430 AND post_type=3 ) 
  UNION SELECT id, 4 as post_type, last_comment_time FROM discussions WHERE id IN ( 
    SELECT post FROM subscriptions WHERE user = 3494430 AND post_type=4 ) 
) posts 
WHERE last_comment_time > 0 ORDER BY last_comment_time DESC LIMIT 0, 4