What are the differences between the How To Reassess Your Chess books from Jeremy Silman? 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.