How it works:

• Downgrades the game to version 1.0.0.0 using binary patches, leading to a (mostly, only difference is launcher and addition of xliveless and a commandline.txt file to allow the game to launch) perfectly replicated 1.0.0.0 version (this is mostly for my own convenience to only maintain one set of patch files, but also to give the user more options for which version to install)
• (if enabled) Install the selected patch by using Rockstar's official installer.

It supports installing all patches from 1.0.1.0 up to 1.0.8.0 (including the Russian and Japanese versions).

Before using this I recommend verifying your game files on Steam to make sure you have all the original files. The program automatically makes a copy of the game in case something goes wrong so you're not left with a non-functional GTA IV install.

My testing so far has not been super in-depth, so I'm releasing this as a beta. If any problems crop up, please let me know.

GitHub repository with all the code involved: https://github.com/hoXyy/IVCE_Downgrader