Two unaired episodes exist: The last episode of Series 3 and the last episode of Series 4. I personally haven't seen them out in the wild yet.
Actually there are four episodes. The show got chopped after 4x03, so there's the final from series 3 and epsisodes 4, 5 and 6 of series 4.I think there were 3 episodes unaired.
The 4th from series 3 (s03e04)
and the final two from series 4 (s04e05, s04e06).
The unaired episodes showed up on usenet in the last two weeks.
You can find them with binsearch.info