The Well-Trained Mind, 4th Edition was published by WW Norton & Company, Inc. WW Norton does not provide a separate booklet or webpage which contains just the revisions.
The revisions are not necessarily to correct mistakes, but rather represent new work by the author to find the best and most up-to-date resources available. The basic "how-to homeschool" information is the same from one edition to the next.
The new resource recommendations are extensive. If you wish to buy the latest edition of the book, see recommendations that were not included in the latest edition, or updates that have come along since its publication, you can find that information here: The Well-Trained Mind Extended Guide.