  It is shown that a sufficiently strong four-spin interaction in the spin-1/2
spin ladder can cause dimerization. Such interaction can be generated either by
phonons or (in the doped state) by the conventional Coulomb repulsion between
the holes. The dimerized phases are thermodynamically undistinguishable from
the Haldane phase, but have dramatically different correlation functions: the
dynamical magnetic susceptibility, instead of displaying a sharp single magnon
peak near $q = \pi$, shows only a two-particle threshold separated from the
ground state by a gap.
