User blog comment:Ḡwẵine Ḹٍٍkƨ Ĺiĸe Ͼềлȑềd/Series 5 will be the last! - unless we act now!/@comment-5220958-20120908105930/@comment-4190137-20120908114506

I think the fact that he used the word "ever", can imply that it be Merlin's last.