Sorry if this has been pointed out already. Didn't read the other answers yet.
shipping is rated on a 1-5 scale, with 5 being defined as excellent. Although the lower scores are not defined with a term, you can assume the lower the number, the worse the experience, with 1 being the worst experience.
I would only leave a 5 if I felt I received excellent service. Anything less and you should be subtracting points. In a case where we all have real lives outside of trading cards, I would expect someone to warn you first before delaying or keep active communications if they have not sent.
4/5 is 80%, which is a B in most grading scales. Nothing really wrong with a B, but it's not an A. However, the scale here seems a bit tougher and dropping below 5 is a bad thing. All the more reason to make sure you send promptly!