Thanks guys for the comments. Indeed, GTA IV vanilla looks very bland in contrast to the vibrant environments of San Andreas and Vice City. ENB series mod changed that, with just some files and a good config, things look incredibly good. Photorealistic, even!
You can get some nice custom ENB configs here in this thread:
For example: Config by Dpeasant3
All you need to do is copy the files in the root GTA IV directory, making backups in case you need them.
You can enable "anti-aliasing" if your card can handle it (my 460 can't) - it's basically downsampling the entire game from a resolution twice your own to your native resolution. An 800x600 display for example would require your card to render GTA on 1600x1200 to achieve "2x" AA. Taxing on your GPU, I know.
Alternatively, you can use this installer to ease things out:
Custom ENB by pedramramfar11
The ENB mods work best on GTA IV patch 22.214.171.124 as it is the easiest version to mod. You could try though to use it on the latest 126.96.36.199 patch, but usually this results in overexposure from the skybox and too much reflection in objects.
The cars on the other hand are just downloaded from various GTA modding sites. Essentially you have to mod the existing car roster in an image file with custom vehicle models and stats. There are tutorials on how to do this like this one: /watch?v=nObJWyF4PVU&feature=related (Youtube)
Have fun and let's see how your GTA IV looks!