Let's get one thing clear from the start: when I rate Shakespeare, I rate it against other Shakespeare; otherwise, the consistently high ratings would not be very informative. If I was rating this against the general run of literature available, it would unquestionably rate 5 stars.
So what brings it down to 4, as compared to other Shakespeare? Primarily a few places where it demands a bit too much suspension of disbelief; the language is some of Shakespeare's best, and is comparatively easy for a modern reader (I found most of the footnotes to be sufficiently unnecessary to be actually more distraction than help). But for one thing, if Richard is withered, hunchbacked, and deformed, how is it that he has been able to kill so many of his victims in battle? For another thing, is it REALLY plausible that Princess Anne would be persuaded as she was by someone with nothing more going for him than Richard? To paraphrase the scene,
Anne: You killed my husband and his father! I hate you I hate you I hate you!
Richard: But I only did it 'cause I'm hot for you, babe! Wanna marry me?
Anne: Welll...maybe. Let me think about it.
(And, in fact, she marries him. Just like that.)
Also, there are virtually NO characters in this play that are sympathetic, save perhaps for the two murdered children and Richmond, and we really don't see enough of them to feel much connection; it dilutes the effectiveness of the portrayal of Richard's evil when almost all of the other characters are, if not just as bad, certainly bad enough.