If You still have problems playing GTAIV on Windows 8, please folow these steps:
1 - (optional) Reboot your PC.
2 - Uninstall the game and then reinstall it.
3 - Do the date check but do not log yet through the laucher. Instead of that, hit the cancel.
4 - Download the lastest patch, unpack and update the game.
5 - Again, the game will launch but you shoud hit cancel again.
6 - Now, the trick for playing under Windows 8:
a) Open the folder where the game is installed, it shold be at "C:\Program Files\Rockstar Games\Grand Theft Auto IV\"
b) Look for an executable named "GTAIV.exe", then right-click it and select it's properties.
c) Under the "Compatibility" tab, choose for running the game in compatibility mode. This action will enable an option to select which Windows version it will run.
d) Select "Windows XP (Service Pack 3)" or "Windows Vista "(Service Pack 2)" because Rockstar DIDN'T EVEN MADE THE GAME COMPATIBLE WITH WINDOWS 7! HOW DO THEY THINK IT WOULD RUN PROPERLY UNDER WINDOWS 8????".
e) Sorry about that... now run the game and have fun with your friends.