This book is awesome. It includes 100% the words and grammar from HSK4, 50% of that for HSK5 and 25% of that for HSK6. i.e, if the rumors are true, and the HSK is going to become more difficult in 2021, then even though this book doesn't probably contain all the vocab that's going to be introduced in the new HSK4, it could very well still serve a good purpose.

It has 2 texts per chapter, LOTS of grammar explanations, exercises of various kinds, and the answer key is on the back of the book. I couldn't confirm if it included the answers before I bought it, so here it is, in case anyone else was wondering.

To me, this book is way better than the theory books from HSK standard course series, I own all the books for HSK3 and HSK4 since I bought them very cheap when I was in china, and although these are specifically oriented towards the HSK exam, this series of books don't feature answer keys, you have to download it from their site, and even so, it doesn't really include all the answers for some reason. Also the grammar points are not very well explained, This is why I bought this book, and I'm very satisfied with it.