Yes, GTA IV is compatible with Windows 8.
Yes, GTA IV is compatible with Windows 8.
If it isn't built-in, you should be able to use compatibility mode ;p
It functioned well for me without any issues. You might need to install gfwl. There could also be a memory-related problem if your GPU has more than 2GB of VRAM. Follow the instructions provided with the product. It worked for me.