This kind of situation is rare for you, unless you are not a full-time student, it may be adult education. Because of full-time students, normal universities or normal colleges are all four-year programs. You have borrowed books for four years. Did you borrow books from the beginning until graduation? When you graduate from a regular university, you will be issued a diploma as long as your grades meet the requirements. Even if you are an adult student and forget to return the book, you will still issue the diploma if you return it.