Unless you've been living under a rock these past few days, you probably already know that Tracy is not dead. Writer Bryan Fuller has already confirmed that Heroes won't be doing yet another cliche sci-fi storytelling element with clones so it's safe to assume that the Ali Larter we saw in the "Redemption" volume teaser is not her sister Barbara. And if you've seen the season 3 finale, you'd know that she's definitely back with a vengeance.
Last time we saw Tracy was in "Cold Snap," in which she uses her freezing ability to kill the agents and, in the process, sacrifices herself to help Micah escape just like Niki Sanders did at the end of volume 2. Danko arrives and shoots the frozen Tracy, causing her to shatter into many pieces, although a fraction of Tracy's face is shown, where her right eye can be seen blinking.
As Heroes heads to season 4, it seems that Tracy is more powerful than ever. She now has the ability to take liquid form and she seems to have found a new perspective after sacrificing herself to save Micah, who obviously made a big impact on her. Retribution may be the only thing on her mind and my guess is that her convictions are even more unwavering than ever, making her more daunting because she believes what she's doing (hunting and killing former agents of Building 26) is for the greater good.
I'm actually uncertain about labeling Tracy as the new villain of Heroes season 4. In the first place, she never really thought of herself as part of the "good" side. She strikes me as a morally gray character and she's also the type of person who would always choose herself over others - at least before sacrificing herself to save Micah.
Maybe Tracy won't turn completely to the dark side but I think having a major female baddie on Heroes is a refreshing break from its previous string of male villains, including Sylar, Adam, and The Hunter, among others. Here's hoping that volume 5, Redemption, will actually live up to its title.