  We propose a new type of magnetically disordered ground state for a
frustrated quantum antiferromagnet. This disordered state is an array of spin
singlets spontaneously formed on four spin plaquettes. Both perturbation
results and bond-operator calculations show that this phase has lower energy
than the columnar dimer state. Analysis of available numerical data on finite
clusters also supports the conclusion that this state is realized at
intermediate frustrations.
