This is a high quality quadruple pump set where three pumps run to cover the duty flow and the fourth acts as a standby.
Another possible configuration for a twin pump booster set is duty / assist. This will reduce the size of pump required and still provide some level of backup (albeit at reduced flow) in the event of a pump failure.
Visit the Carter Pumps website for more information on DUTY / ASSIST / ASSIST / STANDBY Water Pressure Booster Sets