In my personal opinion stop and frisk without a probably cause or reason of suspicion that the person committed a crime is a violation of the 4th amendment but the law doesn't see it that way.
So the police can stop and frisk you and it's not considered to be a violation of your 4th amendment.
In 1968 the law decided that it was non unreasonable for the police to do a terry stop and stop and frisk you without a reason.
So the police can make up stories on why they wanted to stop and frisk you and when the police stop someone because they think they may have broken a law then they have the right to stop and frisk and search you for their own safety.
The police doesn't know the person so they want to make sure the person has no weapons that could harm them when they are talking to them.