I am in agreement with Dr. Hudson re: imdb but Amazon is correct: my box of "Set 4" aka Season 4 has four episodes: Invasion, Bad Blood, Bleak Midwinter, Casualties of War -- yet imdb lists only Bad Blood and Bleak Midwinter, and describes "Season 5" as Bleak Midwinter and Casualties of War, thereafter the "season" or "set" numbering is skewed -- my box of Set 5 includes: Plans of Attack, Broken Souls, All Clear; Set 6: The Russian House, Killing Time, The Hide; set 7: The Eternity Ring, The Cage, Sunflower. All were published via Acorn Media. It appears to me that imdb lost track at Season/Set 4 and has perpetuated the error thereafter. Suggest you investigate and correct if necessary.
The question is whether Set 4 is the equivalent of Season 4. The standard UK term is Series, so I would have expected to see Series 4 rather than Set 4.
If you look at the air dates, Season 3 has 4 episodes in Oct + Nov 2004 Season 4 has 2 episodes both in Jan 2006 Season 5 has 2 episodes in Feb + Apr 2007 Season 6 has 3 episodes in Jan + Apr 2008
Basically IMDb defines the season according the the break between Episodes. When you have Episodes in the 1st quarter of the year, then nothing till the next year, IMDb considers this a season break.
My guess is that the 2 seasons (4 & 5) were combined into a single 'Set' because the company felt that people would be more likely to buy a 4-episode set than one with just 2 episodes, at least at the usual price.
IMDb bases its data on the original showings; therefore, unless the particular show debuts on DVD, the DVD release is totally immaterial to IMDb.
