David Suchet (“Henry VIII,” “The Way We Live Now”) returns in his signature role as suave Belgian super sleuth Hercule Poirot in two new mysteries, “The Big Four” and “Dead Man’s Folly,” based on the novels by Agatha Christie. Whether he’s on holiday abroad, taking a countryside break or simply going about his business near his central London home, Poirot finds himself exercising his “little grey cells” by helping police investigate crimes and murders, whether they ask for his help or not.