Does the Fourth Amendment say that police need a warrant to arrest me?
No, the police officers don't always need a warrant to arrest you. The officer must have probable cause to initiate an arrest, but the need for a warrant might not be present. For example, if a police officer sees a person commit a felony in public, the police officer can arrest the person immediately. When no warrant is present for an arrest, the officer must still be able to show probable cause for the arrest. That probable cause is also necessary for prosecution on the charges to occur.
Do police officers always need a warrant to search property?
Search warrants aren't always required if an officer wants to search property. If you give consent for an officer to search your property, such as a vehicle, home or bag, the search can proceed. If you are arrested for a crime, a search warrant might not be necessary. If contraband is in plain sight, no warrant is needed to search and seize that contraband.
There are several factors that can affect whether a search or arrest is lawful or not. If you have been arrested or if your property has been searched in connection with a crime, going over the circumstances of the situation might enlighten you as to whether the search or arrest was lawful or unlawful.
Source: FindLaw, "Probable Cause," accessed July 15, 2015