The correct answer is sentence 4: "He had come to understand what few doctors at that time did: that bodily functions are affected by spiritual and emotional phenomena as well as by physical influences." In this sentence the author is expressing the main idea of the paragraph in which emotions affect body functions.
The opening sentence in the paragraph gives an example of a doctor's experience. Sentence 2 and 3 provide the context and how people would not agree on the doctor's examining results.
Therefore, sentence 4 is putting together all the details and background in order to state the topic of the paragraph.