  We study the exchange physics in high spin Mott insulating systems with
$S=3/2$ which is realizable in ultracold atomic systems. The high symmetry of
SO(5) or SU(4) therein renders stronger quantum fluctuations than the usual
spin-1/2 systems. A spontaneous plaquette ground state without any site and
bond spin orders is rigorously proved in a ladder spin-3/2 model, whose
topological excitations exhibit fractionalization behavior. The generalization
to the SU(N) plaquette state is also investigated.
