The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no Warrants shall issue, but upon probable cause, supported by Oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized.
Basically government agents of force cannot search you or take your stuff unless they've convinced a judge to allow them to look for a specific thing. No open investigations allowed...just permission to look for specific evidence.
But now the Obama administration continues the hope and change express started by the Bush administration. They argue that warrantless tracking (of cell phones) is permitted by the constitution because American's "enjoy no reasonable expectation of privacy" when it comes to the tracking of their cell phones. They continue to argue that any information about yourself that is stored by a 3rd party (bank, school, doctor, church) is not protected by the 4th Amendment and that the government has every right to it.
The issue is before the 3rd Circuit Court.