AFAIK, the driver software for both Nvidia and AMD/ATI can't force non-post processing AA in GTA IV due to how GTA IV's graphics engine was made (or rather, how it was ported to the PC from the console platform).
You can however, force post-processing AA on it, like FXAA (in Nvidia driver software settings), Morphological Anti-Aliasing (MLAA; in AMD/ATI driver software settings) or SMAA (which I believe provides better visual quality than the former post-processing AA methods). SMAA can be used in GTA IV through injectSMAA:
- adds "Subpixel Morphological Antialiasing" to an application
- is based on "injectFXAA" (written by "some dude")
- is supposed to work only with directx 9, directx 10, directx 11, x86 applications
- may be incompatible with any other form of antialiasing
- may be incompatible with overlays (Steam Overlay, MSI Afterburner, Fraps, ...)
SMAA is a very efficient GPU-based MLAA implementation, capable of
handling subpixel features seamlessly, and featuring an advanced
pattern detection & handling mechanism.
Screenshots (from http://mrhaandi.blogspot.com/p/injectsmaa.html):
Click image previews below to view higher-resolution, original-sized image.