From a pure aesthetic, fun-time perspective, Kost is strong - perhaps one of the best in the city, especially during a summer evening.
Kost's business model is clear: they are optimizing for "vibes", or ambience. This draws in a certain clientele - those for whom food quality is a secondary concern.
With that in mind, do take the average Yelp review in context. While Yelp reviewers are a fairly heterogeneous group, the median reviewer would likely think of food quality as their primary concern.
The food quality here is ok, and I was disappointed by the menu. I was under the impression that Kost's menu was Baja-influenced - of which I only saw sparse representation.
If I were to judge every restaurant under the same context, Kost would likely be a 2-3 star restaurant. But more than anything, Kost provides a dining experience - not necessarily one that is constrained to the quality of its food. And for that, I give it 4 stars.