It's fair use if the work is "transformative". GPT-4 isn't publishing the content of the books, it's publishing a model derived from the entire corpus. I'm not a lawyer, but I think there's an argument that it is transformative.
It's correct that OpenAI isn't publishing any of the "stolen" content directly. But they "stole" it to make their service possible in the first place. Not distributing it themself doesn't make much difference than.