Complement conjecture for the Coble fourfold moduli locus

Let $\mathcal M_4^{(1)}$ be the moduli space under consideration, let $\mathcal M_{\mathrm{Coble4}}^\circ$ be the open locus obtained from smooth stable Coble type quartics, and let $\mathcal D_2$, $\mathcal D_6$, $\mathcal D_8$, and $\mathcal D_{12}$ be the four HLS Heegner divisors. Complement conjecture. Assuming the stability conjecture, the complement $\mathcal M_4^{(1)}\smallsetminus \mathcal M_{\mathrm{Coble4}}^\circ$ is the union $\mathcal D_2\cup \mathcal D_6\cup \mathcal D_8\cup \mathcal D_{12}$. The source also recalls that the complement of $\mathcal M_4^{(1)}$ in $\mathcal P_4^{(1)}$ is $\mathcal D_4\cup \mathcal D_{16}\cup \mathcal D_{20}$. This describes the expected boundary of the Coble-type moduli locus, conditional on stability.