The 4 stars have to do with the purpose of this collection, as well as the variety of compelling stories.
The book is a compilation of short stories for advanced students in English, published by Cambridge University Press. That is to say, it is a book meant for class, for teaching. why is that important? It is important because it appears that in the '70s, the educational system was concerned with tackling many social issues through literature, using texts that today would be big no-nos, whether for being SF, or just plain blunt.
There's a story about the forced Europization of Aboriginal Lands, another about South Africa in the early to mid-20th century, The Pig-Man is a soft study on both prejudice and parents not paying enough attention, and Ray Bradburry masterfully depicts the doom of a society too engrossed in its routine and not paying attention to a shifting of behavior in it children; the Story-Teller bashes the most elementary form of bad parenting, while The Breakout could well be a forerunner to the Matrix, without so much as one stray machine or cable in its narrative.
In short, these were and still are stories to make you think, especially when considering when they were written and compiled and the fact that they were to be used for teaching.