  We consider an anisotropic version of the $CP^{1}$ model which describes
frustrated quantum antiferromagnets with incommensurate spin correlations.
  We extend the two-component spinon field, describing lattice spins, to the
$M$-component complex vector, and show, in the $1/M$ expansion, that for
arbitrary small incommensurability longitudinal and transverse stiffnesses tend
to the same value as the system approaches the quantum critical point. For
physical spins ($M=2$), this yields $O(4)$ critical behavior. However, if the
spin structure is commensurate, the longitudinal stiffness is identically zero.
In this case, the critical behavior is the same as in $O(3)$ sigma model. We
show how the critical exponents interpolate between $O(3)$ and $O(4)$ values
near the transition. We also show that the competition between these two fixed
points leads to a confinement-deconfinement transition at a finite temperature.
