The Riemannian geometry of the regular collapsing regions is actually completely understood. For each $t\in\mathcal{R}_{\epsilon}$, $F_{\beta}^{-1}(t)$ is a $3$-dimensional Heisenberg nilmanifold if the $S^{1}$-bundle is nontrivial, and is diffeomorphic to $\mathbb{T}^{3}$ otherwise. Furthermore, the universal cover of a regular preimage $F_{\beta}^{-1}(t_{j}+\epsilon,t_{j+1}-\epsilon)$ converges to a hyperkähler manifold $(\widetilde{U}_{\infty},\tilde{g}_{\infty})$ with a Heisenberg or Euclidean group of isometries according to whether the $S^{1}$-bundle is nontrivial or trivial. An explicit expression for $\tilde{g}_{\infty}$ in the Heisenberg case may be found in Section 2.2. In particular, our construction gives a concrete example of Lott’s recent work classifying the regular regions in collapsing $4$-manifolds with almost Ricci-flat metrics (see [Lot17] for more details).