Nope. They were made as extended trailers and tasters for Season 5, which aired a long time after Season 4 so the creators wanted to give the fans some extra stuff.
According to Simon, they had an option to talk to HBO about a sixth season, but he decided against it as he felt there was nowhere left for the show to go, so ended it on his terms. Although he was annoyed they only got 10 episodes to wrap it up in.
Thanks for your reply. I'm glad they decided to quit while they were ahead, rather than milking the franchise until it becomes a walking self-parody. I always thought the fifth series was the weakest (still great, mind) - though that is only compared to the outstandingly high quality of the preceeding four.