I think it be a good idea if when am in a big shootout it be nice to go back of my police truck and pick a selection of weapons to aid me in such a fight, cuz i dont know if this is true but dont police cars also carry a assault rifle back of the truck?

It depends on the department. Some Police Departments don't have them for standard patrol (NYPD being one, Boston Police is currently battling city council to have 2 cops in each district armed with them each shift), others have limited amounts (most cities, LAPD being a major one) and some PD's, mainly suburbs, sheriff's offices in rural areas and state police agencies have them as standard issues. As far as where the weapon goes, it depends on the department. Most keep them with a shotgun between the seats, others keep them in a rack on the ceiling of the car, some keep them in the trunk and the Kansas Highway Patrol keeps them in the back seat oddly enough (with a rear partition separating suspects from the guns).

 

As far as GTA 4, go to LCPDFR.ini, then to the setting called "VehicleTrunkWeapons." You will then need to put the ID down (full list in the LCPDFR manual, page 28, section 5). You are then free to choose whatever weapon to be put in the back trunk