  Let $g : X \to Y$ be the contraction of an extremal ray of a smooth
projective 4-fold $X$ such that $\dim Y=3$. Then $g$ may have a finite number
of 2-dimensional fibers. We shall classify those fibers. Especially we shall
prove that any two points of such a fiber is joined by a chain of rational
curves of length at most 2 with respect to $-K_X$, and that $|-K_X|$ is
$g\text{-free}$.
