(2)

Generally, the halides of group-14 elements are covalent in nature. ${\mathrm{PbF}}_4$ and ${\mathrm{SnF}}_4$ are exceptions which are ionic in nature.

(1)

${\left[{\mathrm{SiCl}}_6\right]}^{2-}$ is not stable due to steric hindrance by large sized $\mathrm{Cl}$ atoms.
 

(4)

The inertness of $s$-subshell electrons towards bond formation is known as inert pair effect. This effect increases down the group thus, for Sn, +4 oxidation state is more stable, whereas, for Pb, +2 oxidation state is more stable, i.e., ${\text{Sn}}^{2+}$ is reducing while ${\text{Pb}}^{4+}$ is oxidising.