Hey mate, this is pretty good work. I am guessing you enjoy detail? :P
Your model is good and you seem to have a firm grasp of creating all the shapes. It is clear to me though that what you need to work on the most, is exactly how you spend your polygons. It should be perfectly possible to retain the level of detail you have here, but with much, much less polygons. I mean there are a lot of areas I could pick on, but take the door hinges and the rubber grommets for the door wiring as a single example - that is way overkill dude. You could make a dead simple mesh there, and then just normal map the smoothness back into it. In fact it may make it look even better, since there is no polygon limit when creating your normal maps. I realise that normal maps are not suitable in every game engine or situation (you seem interested in GTA4 which I am not familiar with), but it is something to consider.
Also one other thing I would like to pick on is that you modelled the brake calipers at the front almost as thin as the discs themselves. That is really bizarre to me. Take a walk outside and look inside the wheel of the first car you come across please!
In your last images they seem to have disappeared completely. I hope you didn't delete them in an attempt to save polygons. They really should be there, even if simple. Calipers are very easy to fake detail in the textures with because you typically see them from a straight on angle and from above, so that the shadow almost always looks correct i.e. if you saw the car lying on it's roof, then only would the static lighting would break down really noticeably.