This should be watched AFTER the Season 4 episode "Sometimes A Great Notion".
It was written by Jane Espenson and Seamus Kevin Fahey, and was directed by Wayne Rose. It won the Streamy award for best Dramatic Series, but has not been included in any official Battlestar Galactica releases, and is unavailable to buy online.
If you are based in the US, you can watch all 10 episodes at Hulu.com for free. This version combines all 10 episodes into one long one, without anything omitted.