  Let $M^n$, $n \in \{4,5,6\}$, be a compact, simply connected $n$-manifold
which admits some Riemannian metric with non-negative curvature and an isometry
group of maximal possible rank. Then any smooth, effective action on $M^n$ by a
torus $T^{n-2}$ is equivariantly diffeomorphic to an isometric action on a
normal biquotient. Furthermore, it follows that any effective, isometric circle
action on a compact, simply connected, non-negatively curved four-dimensional
manifold is equivariantly diffeomorphic to an effective, isometric action on a
normal biquotient.
