Downgrading GTA IV to work on GTA Connected

Instructions

Use the tool Steam provides to verify integrity of game files so they get converted back to their original state.
Then, download the patch for either File:IV-Downgrade-1.0.7.0.zip or File:IV-Downgrade-1.0.8.0.zip, depending on which version you want to use.
Unzip it. it should make a folder called Retail.
Drag the Retail folder into your GTA IV game directory. Don't move the files out of the folder! Just move the whole folder itself.
Open GTA Connected launcher and select Tools > Game Settings and choose GTA IV from the game select dropdown.
Use the Browse button to set the path for the exe to GTAIV.exe inside the new Retail folder in your IV install directory.
Launch the game and enjoy!

If you've followed those instructions correctly, the original files should be intact and unmodified. This means Steam won't force the files to be reverted, and you can now launch the patched version using GTA Connected or also use the original through Steam without any further changes*