User blog comment:Ḡwẵine Ḹٍٍkƨ Ĺiĸe Ͼềлȑềd/Series 5 will be the last! - unless we act now!/@comment-5102537-20120909135605

I think this interview almost confirms what the tweet was about, meaning that they will focus on the movies after season five. It doesn't sound to me like another season plus three movies will be done:

http://www.cultbox.co.uk/news/headlines/4216-merlin-film-trilogy-planned-could-reboot-whole-series