The 4th amendment and surrounding case law deals with two distinct issues (actually, more than two, but two relevant here): that some searches require warrants and some do not, and that for those that do require warrants, probably cause must be shown in order for said warrants to be obtained. "Reasonable expectation of privacy" is the standard for deciding what is or is not a warrant-requiring search under the 4th amendment (see, e.g., Katz v. US). "Probable cause" is, given that something is a warrant-requiring search, the standard that needs to be met in order to get the warrant. The former is the one relevant to this discussion, not the latter.
