According to Grand Times:
四皇を止める力として、世界政府は海軍と王下七武海を従える。
The World Government has Marine and Shichibukai as forces to stop the Yonkou.
海軍 - 海上の治安を守る、世界政府機関のひとつ。敵は同じだが、王下七武海とは相容れない。
Marine - One of the World Government's organization that protects safety on the sea. Even though they have same enemy, they are not in friendly term with Shichibukai. (They don't work together.)
王下七武海 - 利益の一部を上納することで、世界政府から略奪を許されている。海賊への抑止力という意味もあり、その実力 も折り紙つきだ。
Shichibukai - By paying part of the profit, they are permitted to plunder by the World Government. This works as a restraint against the other pirates, and their ability is unquestioned.
四皇 - "偉大なる航路"後半の海にて、皇帝のごとき力を誇る海賊。頭は4人だが、その戦力は他の世界的勢力と並ぶ 。
Yonkou - Pirates that shows emperor-like power in the later half of the "Grandline." Even though there are only four of them, their power equals to the other world forces.
7 Shichibukai = 4 Yonkos.
Shichibukai wins every scenario.
