You can't. Mods.
fileplanet
hotswap
Various websites. Try typing gta iv mods for ps3,xbox(etc.etc.)
actually you have to buy the mods once you buy the one you wont its free.
For example with GTA IV, you can modify these files: vehicles.ide, vehicles.img, carcols.dat and handling.dat. that are found in the directory. Just search for "GTA IV PC mods cars" in Google and you will find some fantastic tutorials..
yes
There is no cheat, however if you own a pc version of GTA IV, then you could use a mod, try gtagaming.com for this and many more GTA mods.
No.
there are not any jetpacks in gta4 people use mods and post it on youtube
There isn't a Lamborghini Gallardo in GTA IV. If you play GTA IV on the computer yo can download mods, and then you can download a Lamborghini Gallardo and put it in the game.
Yes it is, but it is limited to some versions of GTA IV, better try this and make sure to read he readme's, it's simple and it works with all versions after "GTA IV filecheckfix": http://www.gtagaming.com/downloads/gta-iv/script-mods/4133