first booking, left 4 stars, should I be disappointed?

Sure, I would be as well. You might want to contact the client to ask what you could have done better to make her dog's stay with you a 5-star one. But I wouldn't push it. Some people, on principle, won't leave 5-star reviews. I had one of those and she told me. The next time her dog stayed with me, I politely asked her not to leave me a review. To me, if a sitter provides regular/daily updates and lots photos and the dog is in the same shape as when dropped off, then the sitter deserves a 5-star rating.

Hi Susan: I wouldn't be disappointed. Some people don't believe in 5 star reviews for whatever reason. Some people don't leave reviews at all, even though they are repeat clients and are happy with your service (and the dog LOVES you). Don't worry, they'll be plenty of other clients that will leave 5 star reviews!