At least 8 million viewers tune in Sunday to season four premiere . That 's up from 6 million at the start of ` ` ' 's '' season three . The season four premiere outperformed every other drama on Sunday night , too . PBS also says the show has become a post - holiday tradition .
At least 8 million viewers tune in Sunday to season four premiere . That 's up from 6 million at the start of ` ` ' 's '' season three . PBS also says the show has become a post - holiday tradition .