How To Reassess Your Chess 1st, 2nd, 3rd, and 4th Edition
What's the difference? Are these improvisations or just parts?
The editions do not form a series. It just is that the 4th edition is the one released at the latest date (October 2010) with revisions to the text.
There is also a workbook which contains a summary of the main book, with about 300 examples with detailed answers. This was released in December 2000. This is the book that I personally found the most useful.
The snarky side of my mind says the reason there are x editions of the book is Silman wants to make money. This is not evil, but really you only need one of the editions. The other editions are pretty much Silman saying the same things using different words and different examples. If you go by the principle "more material means better book" grab the most recent one, whatever it is. It gets longer every time he releases it.