Stability of CI4 and PbI4
Steric hindrance does indeed interfere with the stability of carbon tetraiodide, as does poor covalent bonding overlap between the very differently sized carbon and iodine atoms.  Even so, carbon tetraiodide is definitely established and in use as an iodinating agent.  Storage below normal room temperature is recommended.

Carbon tetraiodide is also very red.  Ordinarily saturated carbon compounds are white or colorless because the electronic transitions involving single bonds to carbon require ultraviolet radiation to go, but the relatively weak bonds in carbon tetraiodide enable transitions in the visible light region.  Iodoform is more lightly colored for a similar reason.

Lead tetraiodide, meanwhile, does not seem to be definitely known to exist.