4.0 is the exact figure CGC has in the guide for a fully detached cover. It actually lists possible up to 5.0, but I assume this means an extremely pristine book that somehow has a detached cover. I don't know how that happens since an unattached (never stapled) cover would get a Qualified grade, and it's hard to detach a cover without some tearing beyond exactly at the staple. I don't have the book in front of me, but as @grendelbo suggests, splits, especially full ones are treated harshly. I vaguely recall the absolute max for a fully split cover is 1.8, but I'm not sure how reliable my brain is. It might be 1.0. Splits and stains are big CGC no-nos