1. A plurality of storage systems comprising: a first storage system including a first controller controlling a first write request received, and a first storage storing data under control by said first controller; a second storage system including a second controller controlling a second write request received, and a second storage storing data under control by said second controller; a third storage system connected to said first storage system and said second storage system, said third storage system comprises: a third controller controlling data received from said first storage system synchronously and controlling data received from said second storage system synchronously, and a third storage storing data under control by said third controller; and a fourth storage system connected to said third storage system, said fourth storage system comprises: a fourth controller controlling data received from said third storage system asynchronously, and a fourth storage storing data under control by said fourth controller.