Ben McKenzie, Shawn Hatosy Ben McKenzie, Shawn Hatosy

Southland's fourth season finale is going to put some of the show's new partnerships to the ultimate test.

Southland: Breaking down the new partnerships

As Tang (guest star Lucy Liu

) readies herself to move on to a new precinct as a newly minted sergeant, Cooper (Michael Cudlitz) isn't exactly ready to let her go with a handshake and a smile. That anger stems from the fact that Tang lied about tampering with a crime scene after she shot a teenager who pointed a toy gun at her."Her integrity is completely f---ing shot with [Cooper],"Cudlitz tells "She made this choice, and now John is wrestling with the fact that it's a mistake. ... He feels that what she has done you cannot come back from. She has crossed the line with no excuse."

Tang, however, won't heed any of Cooper's warnings, particularly since she knows of his own questionable past as a cop who stayed on the streets despite a debilitating painkiller addiction.

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"She's like, 'Who is he to cast that judgment?'" Cudlitz says. "He is, at this moment in time, possibly the king of mistakes. So where do you draw that line as to who gets forgiven for what? He's very aware of what can happen to someone in the heat of the moment... but he is fighting against that. He tries to hold himself to the highest standard, and some choices are made. You'll either agree or disagree with John's choice based on his history."Meanwhile, Ben (Ben McKenzie) will find himself set adrift by a recent accident that threatened the life of his partner Sammy (Shawn Hatosy). In the season's penultimate episode, a pimp Ben had beaten up opened fire on Ben and Sammy's police car. Although they escaped without being shot, their police cruiser was then broadsided, leaving Sammy bloody and unconscious.McKenzie says these recent developments will lead his character to even darker territory. "I would say the guilt and frustration and the feeling of 'nothing's going to get better unless extreme action is taken' sort of has a cumulative effect," McKenzie says. "It tips Ben over into another place. ... You are certainly left to believe that he has made a pretty serious decision that is pretty far outside the bounds of what he ought to do morally, legally, ethically. It's pretty severe. It is a far, far cry from where we started.

Check out photos of the Southland cast

McKenzie says he's excited to see what the repercussions of his character's decisions might be, should TNT renew the series for a fifth season. "I think if we handle it correctly, it'll really allow the character to go in all sorts of really interesting directions, because he really was a blank slate this season," McKenzie says. "To go from the guy who memorized the rulebook to the guy who literally sets the rulebook on fire and takes the world on by himself is a pretty huge change. I haven't seen a show take the golden boy and just spin him 180 degrees and beat the hell out of him and see what happens. I think it'll give us a great place to start next season."Southland airs Tuesday at 10/9c on TNT.