No, not in ordinary gameplay. In GTA IV you are Niko Bellic, and that does not change. However, I do not know if there are mods to change Niko into something else, you would need to have a look at dedicated GTA sites like gtawiki, where there is a comprehensive modifications section for mods that can be downloaded via computer.
Yes the protagonist is different.
there's loads of "drug people" in G.T.A IV. find them in the city
NO
it is about nNiko an ex-soldier from eastern euroup trying to find someone he thaught in liberty city life would be different from when he killed people smuggled people and sold people also he is believed to owe money to ray Bulgarian but that was rumored to be a mistake of Bulgarian
Yes But No Parachute.
NO! never!
Yes
Annihilator
Under the bridge
You can fly helicopters.
press r3
Play the game like any normal people.