As I understand a thesis statement, the choice is between 2 and 4. I think by the narrowest of margins, you have to pick 2. But do not be surprised if it is 4. Twain says that he will talk about one of the ways to tell a story, the humorous story. That comes from one. The problem is that he wanders around talking about wit (French) and Comic (British). But when he gets around it, most of the information is in either 2 or 4 and both indicate a direction. Four looks more like a transition statement. Many of these questions that you have posted are so difficult because you don't know how things develop beyond the quotation. Four contains details of where he is going, but the real meat of direction is in two.
