you're right, There is no texture difference. The only changes are lighting/reflection/refraction and post production filters. Which can be GPU hungry, but it's worth it!
Also the Time cycle has been overhauled i think. They coded their own skylights and light effects for different times of the day and weather cycles.
It's not even a hard mod to install. You just drag - drop the files into the GTAIV route folder, and modify the config according to your setup, and you're ready to go.
Installing GTAIV and getting the v1.0.4.0 patch to work properly is a different matter!
I had to pinch my arm because it looks better than reality!
If I had IV for pc, how would I acquire this mod?
What hardware would I need in order to run this as smoothly as possible? The game is some years old now, I don't know if I would need a high end pc or just a "better than average" gaming pc.
What are your pc specs and how does it run the game?