For OpenIV, I think this might work (probably you need a Google Translator).
It's a tutorial on how to install cars with OpenIV. I think the peds and weapons should work too, but you need to open componentpeds.img or weapons.img instead of vehicles.img.
As for Simple Native Trainer, don't know if it's right, but I have a video:
If it's not right, I'll teach you how. Send me an e-mail at email@example.com
if you do.
EDIT: I forgot about checking the game version.
Open the Grand Theft Auto IV folder (at the directory where you install it, usually for x64 users as a default found at C:\Program Files (x86)\Rockstar Games), right-click the GTAIV.exe, click the Properties button, and click the Details tab. You will find the game version of your GTA IV.