Third homology of a closed oriented four-manifold ($H_3(M;\mathbb{Z})\cong \mathrm{Hom}(H_1(M;\mathbb{Z}),\mathbb{Z})$)

For a connected closed oriented four-manifold, Poincare duality identifies $H_3$ with $H^1$. The universal coefficient theorem for cohomology identifies $H^1$ with $\mathrm{Hom}(H_1,\mathbb{Z})$ because $H_0=\mathbb{Z}$ has zero Ext. Consequently $H_3$ is torsion-free and its rank equals that of $H_1$.