I don't know about GPT 4, but I did not find GPT 3.5 great for summarizing long documents. I tried it on two long that I knew the content on, and it did not pick up the main themes well. Not even well enough that it would be believable at a glimpse, but I would be more concerned if it was believable and used as the basis of a debate. GPT 4 should do a little bit better in that regard, since it allows for longer input strings, but still not long enough to properly process 4,000 pages.