[Dueller]

I'd be curious to know the circumstances of the other three stops: late at night? leaving bar/restaurant? accused of speeding? careless driving? registration/tag violations?

What you are dealing with is a classic 4th amendment question...i.e., right to be secure against unreasonable search/seizure. While the Constitution talks in terms of your home/person and your car is generally seen to be an extension of your home, there is a lowered expectation of privacy when operating a motor vehicle on the public highways. In any event, the officer has to have probable cause to stop or atleast reasonable suspicion a crime is being committed (such as a traffic violation). A brief investigatory stop that is limited in scope/duration/intrusion will likely be allowed under judicial scrutiny. Though terms of art, LEO's will usually convince a judge he had probable cause/reasonable suspicion.