There are several thesis statements that one could craft regarding this chapter. A strong thesis will make a defensible claim about the chapter that can be supported with examples. For example, one of Gladwell's main points in this chapter is that people's upbringings and socioeconomic contexts impact their ability to be successful. A relevant thesis on this topic might look something like this:
In chapter 4 of Outliers: The Story of Success, Gladwell proves that the means to learn practical intelligence is crucial to a person's success.
This would be a defensible claim about the chapter because it argues that Gladwell is making a specific point. A reader might be able to disagree with this claim, though, so it must be supported with evidence. For example, one could explain how Oppenheimer came from wealth so he had the means to acquire intelligence and get around challenges to his success. In contrast, Langan did not come from a high socioeconomic standing. This led to obstacles, like the incorrect financial aid form, which prevented him from receiving direct opportunities to succeed.