  A spin-1/2 antiferromagnetic(AFM) generalised ladder model is constructed
which consists of four-spin plaquettes, coupled through weaker exchange
interactions, to two-spin rungs. In an extended parameter regime, the exact
ground state of the ladder is determined. In this state, the four-spin
plaquettes and the rungs are in their ground state spin configurations. In the
presence of an external magnetic field, the magnetization/site has a plateau
structure as a function of the magnetic field.
