How do you get more than a 3 star wanted level in free mode on GTA 4

It is very easy to get past the 3 star level. By firing at the police will immediately get you a 3 star wanted, after that, just keep lobbing grenades into the traffic, you will soon see 4 stars and past, particularly if your grenades blow up chasing police cars and then NOOSE vehicles. Shooting down a few chasing choppers is always a safe bet for ramping up the stars.