“Vikings” (10 p.m., History Channel): Season 4 of “Vikings” launches, but

“Vikings” (10 p.m., History Channel): Season 4 of “Vikings” launches, but Ragnar (Travis Fimmel) is in no mood to celebrate. He achieved victory in Paris at the end of last season, only to return to Kattegat suffering from a grave illness.

TV listings, B6