If these dogs are roaming, then they have no owner and that is an issue, not the fact that their breed is some type of pit bull.
Pit bulls are not bad dogs, they are only as bad as their owners. Bad people who want to train dogs for fighting gravitate toward the pit bull breed, but that doesn't mean the breed is aggressive. In fact, they are not people aggressive, but they can be toward other animals. But since you don't care, you just wanna know how you can punish the "bad pit bulls" who have "at least 4 attacks in the last year". Pit bulls are dogs, plain and simple.
Why don't you people make laws for all stray or aggressive dogs instead of hyping up the "pit bull" part? Have you ever seen a pit bull attack anything? I doubt it. You've only heard or read it somewhere.
The fact is there is always a breed of dog the general public views as vicious and scary. It used to be Dobermans and Rottweilers. Now it's Pit Bulls. Next it may be German Shepherds and Chihuahuas (very scary!)