The Fourth Amendment will affect police, but it won't restrict a random person who is given access to something from turning over whatever data they want to police.
Say I hire a painter, and the painter is painting my house's interior, and sees a bloody knife in my house. He can report that to the police. But, remove the painter from the picture, and the police could not enter to look for such a thing absent a warrant.
'course, the flip side of that is that if the police get a warrant, then they can enter whether I want them in the house or not, whereas the painter can only enter because I choose to let him in.