While an upgrade to 8.1 or 8.0 would defently make your pc a little faster on the desktop and such, i don't think you'll see any difference in GTA IV.
Though if you want better performance, just searching "gta iv on intel hd graphics" on youtube will give you some tutorials and gameplay videos with their tips in the description. top video when doing that search has a couple of good tips. Here are some tips for you:
1: Install Game Booster if you have alot of programs running on your pc.
2: When you're running the game, go to Task Manager, then the tab "Processes" and set GTAIV.exe's priority to high by right clicking it.
3: make sure that vsync isn't enabled, and this counts for EVERY game that isn't a 2D-easy-to-run-game.
Hope this helped!