This is Volume 4 of the series. It is presented in original font and text. Newly printed. The Department of Defense is releasing for public use and research this multi-volume study giving the "MAGIC" or communications intelligence background of the 1941 Pearl Harbor disaster. In its review of classified records pursuant to E.O. 11652, the Department of Defense decided that it was in the public interest to declassify the intelligence which the U.S. obtained from the communications of its World War II enemies. This study contains a major part of the communications intelligence which the U.S. derived from intercepted Japanese communications during 1941. The documentation presented here is both voluminous and significant. The large volume of intelligence concerning Japanese secret plans, policies, and activities which U.S. cryptologic specialists produced will augment the information already available on Pearl Harbor from Congressional and other public hearings. Of particular importance in this study is the correlation of the intelligence with the discussions of Secretary of State Hull and Japanese Ambassador Nomura in the critical months before Pearl Harbor. Scholars no doubt will find new challenges in this voluminous intelligence information. as they examine not only the decisions made by the U.S. but also the intelligence which influenced and occasionally prompted those decisions.