I have created a thread in the PCGW forums about the game under modern systems. It's not supposed to be an in-depth guide, just a discussion thread so people feel more inclined to share their experience with the different tweaks and fixes for the game present in the article, as well as any issues. I encourage anyone currently playing or planning on playing the game to take a look, and maybe share their experience so that any relevant information can be gathered:
https://community.pcgamingwiki.com/topic/5015-getting-gta-iv-to-run-optimally-on-modern-systems/