By jacking a cop car, Niko can access the cop computer (instructions on screen) to find names of criminals to hunt down in vigilante mode, and gain a reward.
seriously? he's a cop on a motorcycle
NO
Yes But No Parachute.
NO! never!
Yes
Under the bridge
Annihilator
You can fly helicopters.
press r3
If he asks for it, give it to him. If he loses his job, that's one more bad cop off the streets.
No.
If you mean like working in burgershot or something like that, then no. The only "jobs" you can do in GTA IV are working for people by hijacking cars for them, assassinating people for them, that sort of thing.