The 4 is a hyperparameter you can change, though, so you could set it to 10 as well.
The way it works is that it first looks up the N most relevant documents (N being 4 in the default case) in the FAISS store relevant to the question, so it uses distance of embedding vectors for this lookup.
Then it uses GPT3 to get summaries of the 4 entries related to the question and finally all the summaries together with the question will lead to the answer.
In doing so, you can trace the source where the answer came from and can also point to that URL in the end.
When you make N larger it just gets more expensive in terms of your API costs.