A designer wants to create a whisper chamber in the shape of an ellipse. He has a warehouse space with a longest length of 30 ft, which he decides will be the major axis of his elliptical chamber. He determines the best spots for his guests to stand to experience his whisper chamber will be 4 ft from the center of the warehouse space, which will act as the foci. How far out from the center, along the minor axis, should he build his whisper chamber?