May I ask WHY you shouldn't quote the 4th? If you tell them they CAN search your car as soon as they show you a "warrant SIGNED AND DATED BY A JUDGE, supported by oath or affirmation and particularly describing the place to be searched and the persons and things to be seized" they know you know more about the Constitution then they do and should back off.
This election cycle will end up all about (OMG!) the dreaded inanimate object called a gun.
Forget the FACT that people kill people, not guns.
Again... what don't they understand about "SHALL NOT BE INFRINGED"
gotta go I can't risk the rise in BP level.
To avoid giving them the idea that you may be a member of some right-wing militia. In most states, they can hold you and your vehicle in place while they go and obtain a warrant. Since that warrant would have to based upon probable cause, try to avoid giving them any probable cause by what you say to them.
--Boot Hill