I never watched The Office during its network run, but this month, I signed us up for Netflix, and my daughter has been binge-watching it this week. Having made it through seasons 1 - 3, she turned to the menu for season 4. She was surprised to learn that the episodes are no longer 21 minutes long but 42. She skipped ahead and checked S5, only to learn that the episodes had gone back down to 21 minutes.
The Office did its first extended episode (40 minutes) in season two (“Casino Night”). The first hourlong episode was in season three (“A Benihana Christmas”).
Season four is notable for having several hourlong episodes. The first four of the season in fact. The season four finale is also an hour. The remaining episodes from the season are the standard 30 minutes.
Season 4 was during the writers’ strike, so the actual duration of the season was compressed to only a few months. To make up for that lost time, they made a few of the episodes hour long episodes instead of standard 30 minute episodes.
I don’t recall if it was the case for these Office episodes, but a lot of the time these “hour long” sitcom episodes are simply two regular episodes smooshed together with little, if any, plot connections between halves.