There's a useful list on gta4.net. It provides a list of all vehicles with some basic data (top speed, breaking, etc.), and next to each vehicle is a small camera icon which will display an image of that vehicle when you hover over it.
Another useful list is available on ign, select a category to see a page with a list of all the vehicles under that category, their attributes and a screenshot.
A rough idea of the opposition you'll face at increasing wanted levels is given below:
- 1 star: Police officers on foot and in police cruisers will follow you, they'll shoot at you if they feel threatened.
- 2 stars: Police cruisers will spawn fairly aggressively, and they will chase you as opposed to just following. Police officers will fire on you more often as well.
- 3 stars: NOOSE (SWAT, essentially) will begin to spawn, and they'll chase you with the intent to kill. Police still spawn, and they're much more aggressive about smashing you off the road. Police helicopters will begin to give chase
- 4 stars: FIB will join NOOSE. You'll start to see more of the FIB/NOOSE cars here, and fewer police.
- 5 stars: The police have left almost completely by this point, and NOOSE brings out the Annihilators.
- 6 stars: Extremely aggressive NOOSE and FIB vehicles will spawn relentlessly and aggressively try to kill you.
There's a pretty comprehensive article on the GTA Wikia that may be useful as well.
In GTA4, it's no longer possible to enter a Pay n' Spray when the cops have line of sight to you, which means that you'll have to lose any tails before you can get rid of your stars. Helicopters, especially, are problematic.
You can try to lose any cars that are tailing you, and shoot down the choppers, and then drive like mad for the Pay n' Spray. The problem with this is that shooting down the choppers is likely to increase your wanted level and make pursuers faster and more common.
You can also simply try to outrun them, although this can get tricky since the islands are fairly small and the police will set up blockades on the bridges between islands. It's still doable, however.
Getting an air vehicle can help, assuming you can get away from any pursuing choppers.
Finally, if you can get to an area where vehicles don't spawn much or at all, you can sometimes outrun the search radius. Subway tunnels, for instance, if you've got a vehicle that can get in the tunnels, like a motorcycle.
Kiki Jenkins, one of the girlfriends you can meet online, can remove 3 wanted level stars if you call her. Beyond that point, she can't help you, however.
Best Answer
For general traffic spawns, there's really no rules.
What you're seeing isnt completely random though. Two things skew the probability. First, the game has to economize on memory space. So it's more likely to show you a car you're driving or one quite like it, since it rendering the same model is easier. Even similar cars share some assets. Drive around in a sports car, you are more likely to see that car in traffic, or other sports cars. This kinda doubles down a bit; any car that's already nearby has the same effect, whether in traffic or parked.
Second, because the game tries to be somewhat realistic, the area you're in influences the traffic content. Limos aren't likely down by the docks, but trucks are. Rich cars show in rich areas, and so on.
None of this is a hard and fast rule, unfortunately. Only certain parked cars are consistent spawns. If you are looking for a very specific car, you need to find a parked one or trust to luck.