GTA 4 Moding

What program do you use to mod GTA IV?

There is no real program you can go to http://www.gta4-mods.com/ and get them there. Most should come with a read me file to install it.

openiv blender 3d max photoshop gimp go to gtainsider.com

I think you can now incorporate all modding files with nexus mod manager. But I may be wrong...