[quote name='simmias']It's the last 10 episodes. The DVD released a season 4.0 with the first 10 episodes, and a season 4.5 with the last 10. Then for some ungodly reason they did a season 4.5 blu-ray with the last 10 episodes, even though the season 4 blu-ray contains all 20 episodes from that season along with all the extras that are on the 4.5 set, like the extended episodes. And the season 4 set has Razor, too.
So basically, don't buy 4.5 unless you're getting the DVDs.
Someone chime in if I screwed any of that up.[/QUOTE]
The main thing here is timing. The DVDs released first, of course; when 4.5 came out, it got a simultaneous release on BD, a format first for the series. Later on, it was decided that the series should receive proper full season sets on BD. Now the 4.5 BD is outdated but it served its short-lived purpose as a test run on BD.