In the fifth book we see the thestrals for the first time, whereas before they were just horseless carriages. We then find out that you can only see the thestrals if you've seen death, but Harry's parents happened too young for it to count, and he can only see them now because of Cedric. Then why is it that Harry can't see the thestrals at the end of book 4, rigt after he saw Cedric die?