Is recent legislation on wiretapping a violation of the Fourth Amendment and a step closer to Orwell's vision of Big Brother?

No, legislation on wiretapping is not a violation of the Fourth Amendment and Orwell's vision of Big Brother.

Explanation:

Technically, Wiretapping acts would not infringe the Fourth Amendment as wiretapping is not regarded as search and seizure, which would entail a physical search of the property and belongings of an individual. Moreover, it established your Fourth Amendment rights extend only to one's physical property and not to communications or other non-tangible items, and because the people involved are not forced to reveal incriminating details.

However, the interference of NSA in the privacy of people which is mentioned in Orwell's is a subjective interpretation and therefore needs to be seen in that context.