So far as I can see, Carlton's description of this as "the complete collection" is just untrue, and the claim in Amazon's product description page that it contains "all 27 episodes" is also false, and I hope they'll correct it. The set does contain 27 episodes, but 29 were made, so three hours of the show is missing.
It appears that Carlton left out the concluding episodes 7 and 8 of series 4, Ceremony of Innocence (28 April 1998) and Seasons of Mist (5 May 1998). Maybe incompetence, maybe in order to release series 4 as a three-disc set and not a four-disc set. Internet episode guides seem quite unshakeable on this, and there are plot summaries at various places online. So be warned.