Every Case Is Mislabeled

A wine cellar holds four sealed cases with printed labels: "Red", "White", "Red & White Mix", and "Rare Champagne". You are told two facts: every single label is on the wrong case, and champagne bottles are so distinctive that one glance inside any case immediately reveals its true contents (as does a glance at reds, whites, or a mix). Opening a case to peek costs you effort, so you want to open as few as possible while still deducing the correct label for all four. What is the minimum number of cases you must open?