The last patch for the PC version of Grand Theft Auto IV dropped in 2010; six years later and out of the blue, Rockstar has dropped one more.
The update primarily addresses issues that prevent the game from running as it should on modern systems. So, if you experienced issues like not being able to sign in to Games for Windows LIVE servers, anything related to Windows 8 or newer, crashes, or restricted graphics options on new video cards, you should be good to go now. As a bonus, there are a few bug fixes and minor performance optimizations.
Hit the source if you want the full notes; boot the game now to grab the update.