The homology of moduli spaces of 4-manifolds may be infinitely generated

For a simply-connected closed manifold $X$ of $\dim X\mathrm{\ne }4$, the mapping class group ${\pi }_0(\mathrm{D}\mathrm{i}\mathrm{f}\mathrm{f}(X))$ is known to be finitely generated. We prove that analogous finite generation fails in dimension 4. Namely, we show that there exist simply-connected closed smooth 4-manifolds whose mapping class groups are not finitely generated. More generally, for each $k>0$, we prove that there are simply-connected closed smooth 4-manifolds $X$ for which $H_k(B\mathrm{D}\mathrm{i}\mathrm{f}\mathrm{f}(X);\mathbb{Z})$ are not finitely generated. The infinitely generated subgroup of $H_k(B\mathrm{D}\mathrm{i}\mathrm{f}\mathrm{f}(X);\mathbb{Z})$ which we detect are topologically trivial, and unstable under the connected sum of $S^2\times S^2$. The proof uses characteristic classes obtained from Seiberg-Witten theory.