As it has every year, Downton Abbey will air on PBS well after its UK run, launching its fourth season on January 5, 2014. Fortunately for viewers who wait until then instead of wading into the vulgar modernity of the Internet, much of the next season has already been spoiled for you by news of yet more cast departures and additions: We know, for example, that Siobhan “Mrs. O’Brien” Finneran will be leaving, reducing the number of scenes where someone is duped in some petty power struggle by at least 60 percent. We know Tom Cullen will be brought in as a new love interest whom Lady Mary can spend several episodes ignoring while moaning permutations of the word “Matthew.” We know that wild, jazz-loving “Cousin Rose” has a good chance of becoming the “Cousin Oliver” of a new generation. We know the show will make great strides toward diversity by finally recognizing the existence of one black man. And we know that Maggie Smith will see absolutely none of it.