Why was Berlin divided into 4 sectors after World War

World War II defeats Germany as a result of the East and West cold war aggravating, the American, English, and French Soviet embarks from respective benefit, causes to become the competitor by the wartime ally, afterwards, the Soviet occupied area and the Western occupied area stepped onto the different development path respectively, Germany finally split west the America, England, and France occupied area Federal Republic of Germany and Soviet occupied area Democratic Germany.
In this situation, original Germany the capital Berlin's east half Soviet occupied area and west the half America, England, and France occupied area has formed two sets of systems, city two systems.

NOT into 4 sectors,but into 2 sectors ,is it so ? one is socialistic and another is capitalistic sector~~~!