If you're wanting to downgrade to 126.96.36.199 - to use the new visual mods, you can do the following:
Props go to this guy!
1. Download the 188.8.131.52 patch - I got mine from here
. Install the patch over 184.108.40.206.
2. Download RGSG (a Rockstar Social Group tool) from here
. Do not run yet!
3. Find your GTAIV folder (usually Steam\steamapps\common\grand theft auto iv\GTAIV
) and rename it to something else - failure to rename your original folder will result in you having to re-install the game.
4. Run the RGSG tool - this will create a GTAIV folder.
5. Copy the contents from the GTAIV folder into the folder you renamed in step 3 (your original GTA folder).
6. Rename the folder you changed in step 3 back to GTAIV.
7. Install the mod and run the game
I tried using iCEnhancer with the 'normal' Steam version and then followed the steps above and tried again - BIG DIFFERENCE!