Quite simply, this book is indispensable for nuclear engineers, health physicists, plasma and high energy physicists, and anyone else that works with ionizing radiation detectors on a regular basis.
I have taught a lab course for junior undergraduate and beginning graduate students at a university using the 3rd edition of this book, and found its explanations comprehensive and technically sound. When students didn't understand the concepts presented in class, I found later that more often than not they hadn't read the book thoroughly or attempted the end-of-chapter problems. If I get the opportunity to teach a similar class, I look forward to using the 4th edition.
The book has also been a handy resource for my research, mostly chapter 3, which explains counting statistics in a succinct, understandable way that pure mathematics textbooks typically lack.
The 4th edition fixes a lot of the errata from the 3rd edition, but otherwise is largely unchanged. That's not a bad thing, as the 3rd edition was still excellent. There is some new and updated material as well.
I bought the 3rd edition used and later wished I hadn't, as I got a lot of use out of the book and it became damaged rather easily. I got the 4th edition new, and I expect it to be on my shelf for decades to come.