Frank Underwood's actions are so bad that he's effectively more of an outright villain than an anti-hero, yet still we're rooting for him to succeed, if only because it'll make his eventual fall so much sweeter. So it is that, along the way, he's had to conquer various rivals, such as Peter Russo, Heather Dunbar, and Putin stand-in President Petrov. All worthy adversaries, and he's far from done with the latter two. Will they continue to be the main threats to his power in Season 4, or could one of the new characters emerge as a major contender, or just someone who knows about his past and is determined to bring him down?