The exception that proves the rule

The set-up: I have four cards. Each card has a letter on one side and a number on the other.

Suppose I suggest a possible fact about the cards: "Statement#1 - A card that has a vowel on one side must have a prime number on the other."

The four cards are lying on a table, and the faces you can see are:

E K 4 7

Which of the cards should you turn over in order to determine whether Statement#1 is indeed true? (ok, you could turn them all over, but suppose you are extremely fatigued, what would be the minimal amount of work you would have to do.)
( Ready, class? )