This book, in my humble opinion, is more of an introductory to Buddhism basic doctrine than practical guide to meditation, which, to be fair, has already been perfectly covered in his book "Mindfulness in Plain English".
This book covers:5 hidrances, 10 fetters, 5 aggregates, 6 senses and their objects, 32 body parts, 4 body elements, 108 feelings, 10 perceptions, 7 factors of enlightment, 4 noble truth, 8 noble paths...
As the book name suggests, it teaches us four subjects to be mindful of: body, feelings, mind, Dhamma. I am a little confused with the fourth one, some other books render it as mental formation.
I love Bhante G's book "Mindfulness in Plain English", constantly re-read it and enjoy it very much. In a sense, that book changes my life. As a bonus material from the publisher, I downloaded and read his memoir "Journey to Mindfulness", also an enjoyable read. After finishing this memoir, I immediately purchased his other three books. Read this one first, now I will read the Eight Mindful Steps to Happiness.
In short,if you are interested in Buddhism theory, read this book, if you are only interested in practical meditation guidance, read / re-read his first book "Mindfulness in Plain English. "