Two Presidents' names should be on the list too: Ronald Reagan and Bill Clinton.
Reagan initiated the "get tough" approach in the so-called War on Drugs, and his attitude and policies led to a long list of new laws, new jails, and new sentencing guidelines that changed the look of the justice system forever.
Bill Clinton was instrumental in getting a crime bill passed in the early 1990's that put more than 100,000 new police officers on the streets - giving large amounts of federal funding for local police changed the way law enforcement worked. There was also a significant drop in crime in the 1990s, although this could be due to other factors too.
I don't know that I can come up with four, but I can hopefully help some:
- J. Edgar Hoover, head of the FBI. I would say that he had a major impact on the use of publicity and the media in policing. By controlling the image of the FBI, he built its reputation and helped make the police look glamorous and effective.
- William Bratton was the chief of police for New York City during the 1990s. During his time, crime rates fell and some give him credit for this. He was the first to implement the idea that police should crack down on minor infractions as a way of creating an atmosphere of lawfulness. In other words, small crimes lead to big crimes, so crack down on the small crimes. This was very influential and remains so to some extent today.
- I don't know of any names, but perhaps you can find someone associated with the rise of community policing, which was/is a big trend.
- Don't know if this is an acceptable answer, but what about Earl Warren? His decisions in cases like Mapp v. Ohio changed the rules for policing in the second half of the century.