Before you do anything make a backup!

When you have the newest or the Steam version 
from GTA IV than put all files from 
the 1.0.7.0 folder in your GTA IV folder.

If you have version 1.0.4.0 (Older Patch) than
put all files from the 1.0.4.0 folder in your
GTA IV folder.

Credits:
DKT70,MysTer92 and DayL