The Fourth Amendment is a right to the people saying that people are secure. Unreasonable searches will not be allowed, unless the person searching has a warrant.
A real life example of this would be when I was in six grade. There was a lady that came to help some kids with their work and she lost her phone. She claimed that one of us stole her phone, so were going to have to let our teacher check our backpacks. But right before they were going to do it she found her phone under a piece of paper. Even though they didn't check our bags, they were still going to invade our privacy.