1. More character customization. Such as hair, rings, watches, chains, and tattoos.
2. More air vehicles. All IV has is helicopters. San Andreas has jets, biplanes, cropdusters, and a boat-plane.
3. More special vehicles. San Andreas has a train, jumbo jet, house boat, monster truck, semi truck with trailer, bulldozer, hovercraft, jetpack, and a tank.
4. San Andreas allows you to customize your vehicles and add hydraulics, nitro, and a stuffload of other stuff. All IV lets you do is change the color.
5. San Andreas doesn't require Games for Windows Live. GfWL is the biggest piece of stuff I have ever installed on my computer. It doesn't work half the time and you have to log in to load and save your data.
Rockstar Games has really disappointed me with GTA IV. I hope they do better with GTA V.