My understanding of the rule that has emerged from prior decisions is that there is a twofold requirement, first that a person have exhibited an actual (subjective) expectation of privacy and, second, that the expectation be one that society is prepared to recognize as "reasonable." Thus a man's home is, for most purposes, a place where he expects privacy, but objects, activities, or statements that he exposes to the "plain view" of outsiders are not "protected" because no intention to keep them to himself has been exhibited. On the other hand, conversations in the open would not be protected against being overheard, for the expectation of privacy under the circumstances would be unreasonable.
For this case the guy was inside a phone booth rather than out in the open.
There's nothing from SCOTUS on stingrays, but I looked at one federal case and the data being collected was referred to as a document, so that's where the 4th was found to apply.
In turn I don't think I've greatly oversimplified anything.