I have recently finished all the units to this series (bar some extension exercises).
I have reviewed the book for series 3 seperately. Series 4 started as a bit of a labour. I would reccomend working on a coloquial course of Gaelic first before returning to this series for book 4.
After this detour, I started to get some great results for my effort. I was able to form paragraphs on most topics, and received a stilted but descriptive ability in my language with this course.
The book is not grammaticaly heavy. I also noticed that I didn't know the forms of most transitive verbs after finishing it.
I put my Gaelic to the test, talking to native speakers with my new knowledge, and the courses' efforts really showed with great results.
Each unit has a world of the learner section which is a little stale, and dithering.
How far does the course take you? - well, the cover cartoon of book 4 has one character say - another step and we'll be fluent./ Maybe, but that other step isn't this book, it would be the 5th if one were written. This book is really intermediate 2.
I benefitted enormously from the course, and one can guess at the enormous work put into designing it.