Hi Terry Here. This is what I remember.
Well, the 2nd Edition was done back in the 1992 when I was still at ICE full time. It was the update from the old boxed set of 2 books, Emer, and Atlas Addendum.
The 3rd Edition was produced in the 1990's when I was starting as Eidolon Studio, using SW and RM under license from ICE. It greatly expanded the geographical and racial information from Atlas 2. It is spiral-bound in gold, and I think available as a PDF.
Atlas 4 was basically a reprint of Atlas 3, except for a new section on part of western Emer, with a detailed map, and descriptions of places around the Bay of Izar. These were not written by me, but based on my text from my Eidolon Studio book "Emer I"
I will be including this map in the new edition of Emer I. My new editions of Emer I and II will contain a lot of new material.