  For a class of quasilinear elliptic equations involving the p-Laplace
operator, we develop an abstract critical point theory in the presence of
sub-supersolutions. Our approach is based upon the proof of the invariance
under the gradient flow of enlarged cones in the $W^{1,p}_0$ topology. With
this, we prove abstract existence and multiplicity theorems in the presence of
variously ordered pairs of sub-supersolutions. As an application, we provide a
four solutions theorem, one of the solutions being sign-changing.
