This is, of course, a matter of opinion. I would argue that the 4th Amendment is not a serious blow to the 4th Amendment. As you can see in the New York Times links below, however, others would disagree.
The 4th Amendment specifically protects us not against all warrantless searches and seizures but from "unreasonable searches and seizures." So that leads us to ask whether the conduct allowed in Terry is unreasonable. I say it is not.
Terry does not allow the police to simply stop and frisk whoever they would like. It requires that they see a person acting in a way that makes the police officer suspect a crime is about to be committed. The frisk may only be used to detect concealed weapons. It is not a general search, just a frisk for the officer's safety.
The Fourth Amendment does not and cannot take away all police discretion. The Terry rule does require us to trust the police. That is not always the right thing, but it is a reasonable thing to do under the Fourth Amendment. Therefore, the type of searches and seizures allowed by Terry do not constitute a "serious blow to the Fourth Amendment."