4. If a circle with a diameter defined by a chord passing through the focus of a conic section has no common points with the corresponding directrix, then this conic section is ( ).
A. Hyperbola
B. Ellipse
C. Parabola
D. Ellipse or Hyperbola
4. As shown in the figure, $F$ is the focus, and the corresponding directrix is $l$. $AB$ is the focal chord, $M$ is the midpoint of $AB$, and the projections of $A$, $M$, and $B$ on $l$ are $G$, $N$, and $H$, respectively. Let $e$ be the eccentricity. According to the definition of conic sections, we have:
$AF = e \cdot AG, BF = e \cdot BH$, so $AB = e(AG + BH) = 2e \cdot MN$, which means $e = \frac{AB}{2MN}$. Since the circle with $AB$ as its diameter is separated from $l$,
we have $e = \frac{AB}{2MN} < 1$, hence the correct choice is B.