A wide decomposition pressure range of 132 GPa is predicted for PbH4 above which it emerges in very different forms compared with its group-14 congeners. This triply Born-Oppenheimer system is a nonmolecular, three-dimensional, metallic alloy, despite a prominent layered structure. A significant number of enthalpically near-degenerate structures, with exceedingly small energy barriers for distortions, and characteristic instabilities in the phonon spectra suggest that even at very high pressures PbH4 may exhibit both metallic and liquidlike properties and sublattice or even full melting.