To get the negative out of the way, the reason why I don't give the book a five star is because the language used is not all that captivating. It's written by Phelps himself, and he's not a poet.
The four star goes solely on the story being told. No limits is a fitting name to his journey.
Though Phelps' story is not all that different from other stories of extraordinary achievement - extreme dedication, extreme preparation and extreme passion for one's craft - he puts it in the context of what his body had to endure, and the mindset he has that allows him to overcome all to reach his definite goals.
I haven't read many books on sportsmen's journeys to stardom, but I would assume that most of them are not very different to what Michael Phelps describes. That takes nothing away from the value of this book: I read it and felt inspired to achieve my goals and to overcame whatever may need to be overcome. Yes, we have heard the same story before, still purposeful to read about how someone, another human being, went through the prescribed route of dedication and perseverance, and did indeed come out on top - no cinderella story, instead the story of someone who didn't give a damn about what he wasn't supposed to be able to do.