I can't tell you what the difference is between the 3rd and 4th editions, but I can tell you that I used the 3rd edition and a friend used the 2nd edition for board study this year and we both felt more than adequately and accurately prepared using those editions. I've found that, in general, its safe to go with an older edition of a book as long as its not the first edition (which often has errors) and as long as the subject matter isn't constantly changing (physiology would be an example). I'm guessing that the newest edition of BRS phys probably just has different formatting plus some added unnecessary information, and if you already have the 3rd edition just stick with that one unless someone else can tell you a good reason to get the 4th.