  We study the S=1/2 Heisenberg antiferromagnet on a square lattice with
nearest-neighbor and plaquette four-spin exchanges (introduced by A.W. Sandvik,
Phys. Rev. Lett. {\bf 98}, 227202 (2007).)
  This model undergoes a quantum phase transition from a spontaneously
dimerized phase to N\'eel order at a critical coupling. We show that as the
critical point is approached from the dimerized side, the system exhibits
strong fluctuations in the dimer background, reflected in the presence of a
low-energy singlet mode, with a simultaneous rise in the triplet quasiparticle
density. We find that both singlet and triplet modes of high density condense
at the transition, signaling restoration of lattice symmetry. In our approach,
which goes beyond mean-field theory in terms of the triplet excitations, the
transition appears sharp; however since our method breaks down near the
critical point, we argue that we cannot make a definite conclusion regarding
the order of the transition.
