The cops were probably mandated to make the arrest since it was a domestic situation and there was injury involved. The prosecutor, however, could have declined to prosecute. As for four cops showing up, it was probably a slow day and an assault call came over. So they just took a ride over there.
Sorry, I didn’t realize that “I was just following orders” has now countermanded common human decency. I’ll keep that in mind, that there are some mindless robots on the job.