Really 3.5, but I think that's because I think Walton is trying to talk about ideas almost beyond the scope of the book and, while fiction really is the best way to explore the philosophical themes she is after, there are moments where the story and the ideas are in conflict and, by nature of the thought experiment, the ideas take precedence.
It was still a fascinating and really enjoyable read - I just don't quite know what to do with it. (Give it to Talia, naturally).