  This is the fourth and last part of a series of papers on the long-time
behavior of 3 dimensional Ricci flows with surgery. In this paper, we prove our
main two results. The first result states that if the surgeries are performed
correctly, then the flow becomes non-singular eventually and the curvature is
bounded by $C t^{-1}$. The second result provides a qualitative description of
the geometry as $t \to \infty$.
