Think of this hypothetical. Say Google reads your g-mail and tells people you ordered green shirts. Do you have a lawsuit against Google for disclosing your private information? I doubt it. If the information is sensitive for a different reason (e.g. it's about your herpes diagnosis), you might have a suit, but then you'd also have a suit against a friend to whom you told that information in confidence. I.e. it's a suit based on the nature of the sensitive information, not the disclosure by itself.
The idea of this "information trustee" doesn't really exist in the law. Google is treated no differently than a guy on the street you tell to relay a message to your friend. And as for the 4th amendment, as I said, wiretapping aside it's more of a "personal search" protection than a "communication protection." This is a good summary of the relevant precedent: http://www.cs.duke.edu/~chase/cps49s/carnivore-history.html