I have a psychological block I think. If I give 5 stars to a book, it means it's the best ever, that there can never be anything better. That's just too scary to contemplate, so I usually stop at 4.5
Looking at my librarything listing, the last 4.5 book I read was Journey to the End of the Night, by Louis-Ferdinand Céline. Going back in time, it seems I also gave 4.5 to Blood Meridian, by Cormack McCarthy, though I didn't like it as much as The Road, which I read a few months ago.