$\mathrm{PbF}_{4}, \mathrm{PbCl}_{4}$ exist but $\mathrm{PbBr}_{4}$ and $\mathrm{PbI}_{4}$ do not exist…
$\mathrm{PbF}_{4}, \mathrm{PbCl}_{4}$ exist but $\mathrm{PbBr}_{4}$ and $\mathrm{PbI}_{4}$ do not exist
because of
large size of $\mathrm{Br}^{-}$ and $\mathrm{I}^{-}$
strong oxidising character of $\mathrm{Pb}^{4+}$
strong reducing character of $\mathrm{Pb}^{4+}$
low electronegativity of $\mathrm{Br}^{-}$ and $\mathrm{I}^{-}$.
Solution
$\mathrm{F}$ and $\mathrm{Cl}$ are more oxidising in nature and can achieve Pb in (IV) O.S. but $\mathrm{Br}_{2}$ and $\mathrm{I}_{2}$ can not achieve $\mathrm{Pb}$ in (IV) O.S. secondly $\mathrm{Pb}^{4+}$ is strong in oxidising nature and in its presence, $\mathrm{Br}^{-}$ and $\mathrm{I}^{-}$ can not exist.