Although the PC version of GTA 4 has been released several months ago, players still can't use Anti Aliasing. The Rockstar game only offers the option to (de)activate Post Processing effects. This effect consists of several effects that are used simultaneously only: Depth of Field and Motion Blur. Active Depth of Field doesn't just reduce the sharpness in the distance, but also "softens" the textures and shadows near to Nico Bellic. Thus high-frequency (flickering) content is detected and the whole scene appears more settled. Motion blur is only visible during fast camera turns or when driving at high speed.
But what if you want only the Soft Shadows but no Motion Blur? The sharp shadows are fine but a little Motion Blur would be good, too? The Ultimate Graphics Tweak allows you to use the effects separately. You only need to replace some files with modified versions. Just follow the instructions in the gtaforums.