Volume 1 of these books is the best-selling digital photography book of all time and it deserves to be - it has an almost perfect format of good advice, suitable to everyone from beginners to experienced and the ability to teach something useful on every single page. The successful format has lead to inevitable sequels and while the quality has been maintained, the advice has become more advanced.
For book 4 the majority of the content is aimed at DSLR and advanced Compact System Camera users. For example the first three chapters concern flash, either in the studio or the more advanced on-camera (not built-in) flashguns. Owners of point and shoot cameras will learn from the less equipment-oriented tips but these form the minority of the pages.
So, assuming you have a more advanced camera what will you learn about? Besides flash, which is a dark art for many, there are sections on sport photography, using the video functionality of your DSLR, tips for travel and street photography, landscapes and some more of the "recipe for this type of shot" that are so popular in this series.
I particularly like the nuggets of pro advice that are liberally sprinkled across the pages, such as getting better street photographs by sitting somewhere quietly for a while until the people around have forgotten you're there and only then beginning to quietly take photographs.
What I really like about this series is the lack of waffle - there is something of interest on every page and the text gets right to the point. They're also great when you have time to take pictures but are stuck for an idea - just open a few pages at random and something will inspire you. Scott's trademark humour is still present in this volume, which makes it an enjoyable read as well as informative.
Each volume is gradually becoming more advanced, so beginners should start with the first volume and work through in order but otherwise this is a perfect volume 4.