I went looking in the preface for what's new in the 4th ed. - the only thing I found is on the back cover: "The fourth edition has been substantially revised a

I went looking in the preface for what's new in the 4th ed. - the only thing I found is on the back cover: "The fourth edition has been substantially revised and expanded to bring the content up to date with the current Scheme standard, the Revised6 Report on Scheme. All parts of the book were updated and three new chapters were added, covering the language's new library, exception handling, and record-definition features." "The book offers three chapters of introductory material with numerous examples, eight chapters of reference material, and one chapter of extended examples and additional exercises. All of the examples can be entered directly from the keyboard into an interactive Scheme session. Answers to many of the exercises, a complete formal syntax of Scheme, and a summary of forms and procedures are provided in appendixes."