The Fourth Amendment applies to searches and seizures by agents of the government, not by private citizens. (Burdeau v. McDowell, 256 U.S. 465 (1921).)
If, for example, a friend rummages through your belongings, finds contraband, then hands it over to the police, the Fourth Amendment doesn’t prevent its introduction at your trial. If an officer had gone through the belongings without a warrant or probable cause, the evidence probably wouldn’t have been admissible in court. (See What is a motion to suppress?) But since the government didn’t conduct the search or request or encourage your friend to carry it out, the evidence would likely be admissible.
For Fourth Amendment purposes, government agents include:
In some instances, privately paid officers—like security guards—who have been deputized as members of law enforcement may be considered government agents.
Those who’ve had their property searched should seek advice from an experienced lawyer. They should seek advice whether a private person, police officer, or someone in between conducted the search. If, for example, a private person searched your property, an attorney should be able to advise you as to any recourses you may have. That lawyer should be able to explain, for example, whether and the extent to which the law in your state provides protection against private-party searches.