In GTA4 if you steal a police car and drive around in it for a while you are able to get cops to come and sit in the car with you. When you see a taxi walk up to in and kick the side of the car meaning the cab driver will get out and start attacking you. Go inside the car and call for police back up. The driver will then start to walk away but if you kick him again when walking he will attack and the police will turn up and arrest him. During the time when they are arresting the cab driver enter the police car. The back up police car will drive away and the arresting officer will put the convict into the car you are driving. The convict then exits the car and you get one star. Leave the crime scene or cheat to get rid of cops and you will driving around with a fellow police officer.