1. He hasn't been to Japan.
2. He ain't been to Japan.
(Is #2 an informal sentence. Is it correct?)
3. He has not a book.
4. He ain't a book.
(Is #4 an informal sentence. Is it correct?)
1 is correct.
2 is considered non-standard English and actually marks someone as probably illiterate!!
3 would be better written as this: He doesn't have a book.
4 is incorrect.