Display checkboxes instead of radio buttons (Allow the user to check and uncheck ratings).
Do not provide unstarred (all grey stars) as a facet.
Additionally, append with 'and above'. (Implying 4 stars and above - as apposed to 4 stars exclusively, which can be the current assumption)
As with the colour swatches issue - the visitor is unable to 'undo' and therefore unable to view the previous product listings without backtracking and revisiting the page.
It is of no benefit to allow the visitor to filter products without ratings/reviews.