I have done a commandline.txt, made by different commands made by different people, I mixed it all together and made a brand new commandline.txt!
If you want it, press here.
That fixed my lag/stutter completely, I recommend it to everyone, if it doesn't help, unlucky...
I did it and it worked.
HOW TO INSTALL
Download it.
Put it in your MAIN GTA IV folder.
If it worked, have a glass of cola and enjoy your lag free IV.
(Note: If your computer is real bad, or atleast can't make the IV, this MAY fix it, this is not guaranteed, but it will reduce lag a little!)
My specs:
AMD Athlon II X2 P340
AMD Radeon HD 6650M
750 GB HDD
4GB DDR3 Memmory.
Windows 7 Home Premium, 64 bit
Laptop
So this it it.