Rockstar always rolls them out dialed waaaaaay back. Up until GTA V had already been out for a year the mods were still being applied to IV. They had 4K working in that quite a ways back.

I still see a lot of blurring, and the people don’t appear to be optimized but it looks damn good.

Here was IV Redux, assuming same author:
http://www.moddb.com/mods/grand-theft-auto-iv-redux

(warning: autoplay)