I personally don't think there's a difference between 4 and 4.5 or even 1 and 23 when talking about star ratings
For a book to be Good or Excellent or bad etc is all down to your personal subjective taste, and by experience I have found that reviews sites really don't cater for me.
As they are crowd sourced, with a sprinkling of biased reviews, they will always be the lowest common denominator. Also the reviews are only from people who "are the type of people who write reviews" so that is another limitation. What you end up with is just some generic pop culture score which is not really relevent.
Many times I have read an awesome book, then gone to see the reviews and found them to have slated it. The same happens with music, movies and art.
I'm going to use the rating column for my own personal reviews of Authors in General - ie when I read a book by a new author, and like it I will label All their books I have with the same star rating. (I know they can vary widely book 2 book but it will give me a better idea of whether to try another book by that author or try a new author)