Did you try this one? http://www.rockstargames.com/support/IV/PC/patch/index.html
It's a .rar file. I think it should work
I think shadows on very high are good (although could be much better) but I think the amount of memory it uses is ridiculous. It increases about 130MB memory usage here (1680x1050) and if I want to enable it I need to decrease view distance.
Also reflection resolution on very high decreases performance a lot when comparing it to high.
I'm now playing with these settings
And the game looks very good. I think it's much better to play with these settings than with reflections and shadows on very high because it makes the memory usage ridiculously high and reduces the memory for view distance.
This patch is amazing and increases performance very much. I don't know how much FPS, because my game still crashes when I try to use the benchmark tool (gives a error saying "Please restart the game" but it never works illepall ), but it's highly playable and runs kinda smooth. I think Rockstar is doing the right thing, maybe just a few more patches and the game is fixed and running smooth for everyone.