To get good performance on GTA 4 on low end devices, you must:
Set every graphics setting from High to Low
Low Vehicle Density, Detail Distance, and View Distance
No V-Sync or Definition
Low Resolution (800x600)
Tri-Linear Texture Filtering
The game has also been optimized due to mods (ty again fitgirl <3) (im not associated with fitgirl)
Here's a picture that shows the best settings for a REALLY bad pc
This will make the game look worse, but come on you're running gta 4 on a bad pc that's insane usually it never even runs. If you want it to look good, you could set the settings a bit higher, but it will make performance worse.