Americans by the millions tuned in to “Downton Abbey” at the beginning of its fourth season, eager to enjoy the continuation of the saga of the Earl and Countess of Grantham and their household. According to press reports, 10.2 million Americans watched the first episode Jan. 5, catching up on developments since the end of the third season, which ended with yet another tragedy, this time the death of heir Matthew Crawley. But the heir did not leave without leaving an heir, and so the story continues.