Season 3 ended with a bang at Oswald State Correctional Facility: Level Four, as racial tensions reached an all-time high. Now, with inmates still on 24-hour lockdown, prison officials are looking for ways to end the hostilities and return Em City to normal. Truth is, Em City was never "normal." And if anyone thinks the worst is over at Oz, they’re dead wrong. HBO presents the fourth season of this acclaimed drama series that focuses on an experimental "prison within a prison" called Emerald City, which is filled with a now-familiar congregation of colorful — and highly volatile — inmates. With McManus fired and Emerald City under new Unit Manager Martin Querns, things are definitely changing . . . but not necessarily for the better.