The vast majority of mods require version 1.0.8.0 or older of GTA IV, and they're better versions overall.
Here's a quick and easy guide on how to get 1.0.8.0.

Downgrading automatically

• Download ZolikaPatch IV and extract it into your game folder.
• Run the game, it will ask you to downgrade. Press OK.

Manual method (for advanced users)

• Create an antivirus exclusion for your game folder. (Optional but heavily recommended)
• Download the downgrade archive.
• Extract the contents of the .zip into your game folder, replace all if prompted.
• Run vcredist_x86.exe and let it install.
• Run the game.

Note that the manual method will move your savegames!
You'll have to move your savegames from:
Documents/Rockstar Games/GTA IV/Profiles/(profile ID)
into:
%LocalAppData%/Rockstar Games/GTA IV/savegames/user_ee000000deadc0de

Additional notes

• ZolikaPatch and IV Tweaker are already bundled with the downgrader, you don't need to get them separately.
• If you're using the Rockstar Launcher version, you'll now have to start the game via PlayGTAIV.exe in the game folder instead of the launcher, as it auto-updates otherwise.

Optional mods

• FusionFix v1.96 (Downgrade compatible)

Alternate manual archives for advanced users

• 1.0.7.0