This could have several reasons but in case you don't have an Nvidia GPU you can try the "AMD GPU Fix" from the "Troubleshooting & Support" folder.
Even if you don't have an AMD GPU you can try this.


In case you have an Nvidia GPU or if the upper method doesn't help you can try to enable "disable fullscreen optimizations" inside properties for "GTAIV.exe".
(Right click on "GTAIV.exe" > Properties > Compatibility > "Disable fullscreen optimizations")


If both of the upper methods doesn't help:

-Rename/moving your modded GTA IV copy is a potential fix 

-After downgrading make sure to install iCE 3.0 Natural before installing other mods
 and launch your game then to see if iCE 3.0 Natural is really the reason for this issue.

-Check your installed mods. Especially script mods such as .asi, .dll, .cs files are able to cause launching issues
 if they aren't set up properly. Make sure that all installed script mods are compatible with your patch.

-Make sure that you launch your game with "LaunchGTAIV.exe" or "PlayGTAIV.exe" (not "GTAIV.exe") and don't launch your game via Steam if you use patch 4.

