${\mathrm{Pb}}^{4+}$ is less stable than ${\mathrm{Pb}}^{2+}$ due to the inert pair effect. So, ${\mathrm{Pb}}^{4+}$ compounds are very good oxidizing agents. 

${\mathrm{Pb}}^{4+}$ converts itself into ${\mathrm{Pb}}^{2+}$ to attain stability, the process is: 

${\mathrm{Pb}}^{4+}+2{\mathrm{e}}^{-} \to {\mathrm{Pb}}^{2+}$

${\mathrm{Pb}}^{4+}$ is a very big cation and it's association with $4$ anions like ${\mathrm{I}}^{-}$ is not possible. Therefore, ${\mathrm{PbI}}_4$ does not exist and all other molecules exist.