If you are watching the marathon now, you may notice something missing - the familiar jangling opening theme with the four-note repetition. That is because they are starting the marathon with episodes from Season 1, which used a much more low key intro. The more familiar intro wasn't introduced until Season 2.
And season 4 Twilight Zone episodes are the hour long versions. Because season 4 is owned by a different entity - and because of their length - they are seen less often. All the other seasons are owned by one company that does the marathon.