The series has been in a decline in ratings since its premiere. The first season held 13.86 million U.S. viewers in 2006-2007, but the third season fell to a mere 7.61 million.
Several additions as well as removals have been made to characters since the first season. With the most recent episode of having Claire Bennett in a girl-on-girl kiss with Gretchen in order to boost ratings, the step wasn't a successful one. Season 4 is only pulling roughly 5.6 million U.S. viewers, and that's not good news for the show.
NBC and the writers are now forced, it seems, to make drastic changes. This involves one of the main characters coming to an end this season. The character has not been revealed by NBC but certain hints have been.
The character is male in the picture below. He may be present or he may not be, but the character is definitely from the original cast.
Whispers68: If this is true and the character is a male from the original cast, I feel it may be Sylar since he is onto bigger and better things("Star Trek"). But this can be a ploy to take attention away from Hiro Nakamura, who has been facing an illness since last season and is said to be dying from time traveling without fully understanding the consequences. Nathan Petrelli has been "killed," but the situation he and Sylar have been entangled in is weird. Mohinder Suresh has been missing from the current season, perhaps it's he.