The Potsdam Declaration named four principal Japanese islands in its proposed sovereignty limit.
Those islands were Honshu, Hokkaido, Kyushu, and Shikoku. The declaration also allowed for Japan to retain “such minor islands as we determine,” so the number four referred to the principal islands, not every island that might remain under Japanese sovereignty.
The territorial clause was part of the Allies’ plan to dismantle Japan’s wartime empire. It connected the surrender terms to the earlier Cairo Declaration, which had stated that Japan would be stripped of territories it had seized through aggression. Korea, Taiwan, and other parts of the empire were therefore not included in the proposed core territory.
The clause is sometimes simplified as saying that Japan consisted of four islands, which is inaccurate because Japan has many smaller islands and island groups. The precise wording distinguished the four principal islands from additional minor islands selected by the Allies. Japan accepted the declaration through the surrender process in August 1945, and the formal instrument was signed on September 2.