Cavitation is the vaporizing of water and its explosion therein. If the city water supply line only provides 4 gpm, then that is all you should try to pump with your pump. Your pump has the ability to provide some suction—dependent on pump design and elevation, however this suction will most likely not help to create more flow. The most that you can hope to accomplish with this pump is to boost the pressure. Install a 4 gpm flow control and your cavitation problems should be eliminate—if the pump has not been permanently damaged. As someone else state the flow control should be on the pressure side of the pump, not the suction side.
Oh, also, cavitation often sounds like you are pumping gravel.