Sorry guys I guess I should have explained what downsampling is. My gpu is using gpu scaling to render the image at 4K and then it shrinks it to my 1080p screen. This is also the same thing as supersamping AA which you may be familiar with. It provides much better image quality and AA than a regular 1080p screen. Some games that have supersampling built in are the Arma games and Metro: Last Light. So even though the actual pic is only 1080p it was rendered at the detail of 4k. For more information check out these articles:
Here's two examples. These are two 1080p screenshots. The second was rendered at 1440p though or 2560 x 1440. You can notice that the aliasing is less on the lines. At 4k you get even more of an improvement.
Also these are the GTA 4 mods I'm using: IcEnhancer beta 1.25 Better City Textures DKTronics70 road Textures Supreme Vehicle Conversion Pack -=140 vehicles=- Traffic Control System V1.0B DKTronics70 Vegetation First Person mod by C06alt