Rockstar has released patch number 6 for the PC version of GTA IV which features a large number of bug fixes and additional features. The patch is available through Games for Windows LIVE and a standalone downloadable version with be available soon. The majority of these new features/fixes are already present in the PC version of Episodes from Liberty City.
Rockstar Games Social Club
Remove Rockstar Games Social Club application
Separate RGSC application no longer required to launch game
Social Club login now occurs during the launch process
Shadows
A new, less memory-intensive, and better-looking scalable shadow solution has been implemented
Enhanced Night Shadows added with user-selectable level of detail (replaces Shadow Density)
Performance
Rendering optimizations have been made to improve performance, particularly when enabling shadows
User-configurable graphics settings have been added for shadow control
Improved memory management
Multiplayer
Enhanced security to isolate users from cheaters and hackers
Episodic Content
Provide in-game support to purchase "The Lost and Damned" and "The Ballad of Gay Tony" (Not available in Russian/Japanese Versions)
Bug Fixes
Vehicle shadows re-enabled between 8pm and 6am
Fix floating street textures
Pedestrians in vehicles appear further out from the player to allow for long range sniping
The Moon has been implemented with simulated lunar phases
Fixed garbled text in Japanese Multiplayer menus
"RESC 10" error fix for specific hardware configurations
In addition, Episodes from Liberty City players are also getting a small update (1.1.1.0) that will improve memory management and address a hardware configuration error.