Terry Godier argues that App Store ratings are fundamentally broken:

But sometimes they’re impactful in a way that doesn’t match the reviewers intent. For example, if you have a 4.1 star rating in the App Store, any 4 star review is going to decrease that average. In other words, leaving a 4 star review is essentially leaving a negative review.

I agree with Godier’s premise and conclusion, but not totally with the point above. A 4 star rating is still a good one, but perhaps not a great one. So pulling down the average might be justified. Still, other systems are better.