Yes, it is possible, but as far as I know, and as far as my experience goes, nobody has ever tried because there are hundreds of thousands (if not more) textures in the game, and it isn't quite as easy as just replacing them, you would have to see each one (which is possible with the right software) and make a lower resolution texture that looks exactly the same. Basically, think about how long GTA IV was in development, take about a quarter of the staff and a half of the time (maybe a bit more) and that will tell you how long it would take with around 600 developers, then multiply that by the number of developers to show how long it would take for one person to do it.
To give you a short answer, no, not unless you want to retex the whole game.
Regards,
Shaun
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