  A famous theorem of Shokurov states that a general anticanonical divisor of a
smooth Fano threefold is a smooth K3 surface. This is quite surprising since
there are several examples where the base locus of the anticanonical system has
codimension two. In this paper we show that for four-dimensional Fano manifolds
the behaviour is completely opposite: if the base locus is a normal surface,
hence has codimension two, all the anticanonical divisors are singular.
