  The ground state of the four-chain Heisenberg ladder model is numerically
investigated. Hidden-order correlations suitable for the system are introduced
and calculated with an emphasis on the spatially isotropic point, where a
corresponding material exists. The existence of a long-range hidden correlation
indicates formation of a short-range RVB state in the case of the
antiferromagnetic inter-chain coupling. A transition between the phase of the
ferromagnetic inter-chain coupling and that of the antiferromagnetic one is
discussed.
