  We show that a simply-connected closed four-dimensional Ricci flow whose
Ricci curvature is uniformly bounded below and whose volume does not approach
zero must converge to a $C^0$ orbifold at any finite-time singularity, so has
an extension through the singularity via orbifold Ricci flow. Moreover, a
Type-I blowup of the flow based at any orbifold point converges to a flat cone
in the Gromov-Hausdorff sense, without passing to a subsequence. In addition,
we prove $L^p$ bounds for the curvature tensor on time-slices for any $p<2$. In
higher dimensions, we show that every singular point of the flow is a Type-II
point, and that any tangent flow at a singular point is a static flow
corresponding to a Ricci flat cone.
