Definition: Prohibits unreasonable searches and seizures and sets out requirements for search warrants based on probable cause as determined by a neutral judge or magistrate.
Application: Mapp vs Ohio On May 23, 1957, police officers in a Cleveland, Ohio suburb received information that a suspect in a bombing case, as well as some illegal betting equipment, might be found in the home of Dale. Three officers went to the home and asked for permission to enter, but Mapp refused to admit them without a search warrant. Two officers left, and one remained. Three hours later, the two returned with several other officers. Brandishing a random piece of paper, they broke in the door. Mapp asked to see the “warrant” and took it from an officer, putting it in her dress. The officers struggled with Mapp and took the fake warrant away from her. They handcuffed her for being “belligerent.” The police did not find the bombing suspect or the betting equipment. They did find pornographic material in a trunk, that a previous tenant had left behind. She was arrested, prosecuted, and found guilty for possession of pornographic material. (http://www.law.cornell.edu/supremecourt/text/367/643#writing-)
Weeks vs United states On December 21, 1911, Fremont Weeks, the plaintiff in error and defendant, was arrested by a police officer at the Union Station in Kansas City, Missouri where he was employed by an express company. Other officers entered the house of the defendant without a search warrant and took possession of papers and articles which were afterwards turned over to the US marshal. The officers returned later in the same day with the marshal, still without a warrant, and seized letters and envelopes they found in the drawer of a chiffonier. These papers were used to convict Weeks of transporting lottery tickets through the mail. Weeks petitioned against the police for the return of his private possessions. (http://cases.laws.com/weeks-v-united-states)
Katz vs United States Charles Katz used a public pay phone booth to transmit illegal gambling wagers from Los Angeles to Miami and Boston. Unbeknownst to Katz, the FBI was recording his conversations via an electronic eavesdropping device attached to the exterior of the phone booth. Katz was convicted based on these recordings. He challenged his conviction, arguing that the recordings were obtained in violation of his Fourth Amendment rights. The Court of Appeals sided with the FBI because there was no physical intrusion into the phone booth itself. The Supreme Court granted certiorari, indicating it may be interested in reviewing the case. (http://caselaw.lp.findlaw.com/scripts/getcase.pl?court=US&vol=389&invol=347)