Take an hypothetic review of an hypothetic question:
Interesting question ! You made my day ! I have never seen this before.
This is such an opinion based crap that it should never ever been
asked here ...
So sentence by sentence, the evaluation could look like: 1->4, 2->3, 3->2, 4->0. The average would be 2,25 which is a little better than neutral, which obviously is not the case.
The problem is that you apply the rules of continuous related values to something utterly discrete looking, considering a relation where there might not be one.
Unfortunately, there is no good answer here and no magic formula. Here are some guessing mechanics:
- average as you propose
- weighted average giving the sentence more weight depending on their position (Here I'd put more weight on last sentences, as in a conversation, the first sentences are introductory, but for a review it could be possible that the contrary rule is better: the user gives first his spontaneous impression and then try to add more perspective but less gut feeling).
- optimistic or pessimistic approach, taking the either the max or the min of all the sentences.
- averaging positive aspects, average negative aspects, and take the maximum absolute value of either positive or negative, and reduce it by a factor of the value for the opposite sentiment. So 10 minor negative sentiments could be outweighed by 2 extremely positive one.
- same principle, but look at the most frequent positive and the most frequent negative
There are a lot of other alternatives as well, so the first thing that you have to do is to look at 50 reviews, and do your own overall rating, and simulate the different approaches trying to understand why they differ/converge with your own feeling.
If you have enough ressources, instead of trying to find a formula, use some ML approach to let your system learn how to combine the ratings based on your samples.