Complex sentence: It is the seventeenth episode as well as part 2 of the two-hour season finale of the show's fourth season and aired on May 18,2008 in the United States; it aired immediately after "The Gun Song", with no intervening credits.
Split into simpler sentences: It is the seventeenth episode of the show's fourth season, and the second half of the two-hour season finale. In the US, it aired on May 18,2008 immediately after "The Gun Song," with no intervening credits.