Could you reach out and ask "what would make this a 5 star experience for you?". I've done this when people leave 3 star reviews and they almost always want to change the review up a little once they realize that it's not as nice on the receiving end as they seemed to have thought.
The way Etsy describes the star is akin to:
5 stars, It was Great, I loved it
4 stars, It was Nice, I liked it
3 stars, It was OK (I personally think that a C or a 60% is a bad review, but not all people seem to think that way)
My guess is that your customer thought they were doing a nice thing by leaving that 4 star review.